When considering a race's Level Adjustment value—even a race that builds off of another race like the one the question poses—, the first thing to ask is, "Is this appropriate for my campaign?" If you know it is, other questions are inconsequential. (I've offered an analysis here of a prestige class that most folks would say in the abstract is totally overpowered but that would be acceptable—even necessary!—in a very specific campaign; a gorilla avatar of nature may be similar!)
If you're not sure if it's appropriate to the campaign, then you compare it to existing material, ask folks for their opinions, or both. I'm going to do both: I'll compare it to existing material and offer my opinion.
Using official guidelines, the gorilla avatar of nature should have an LA of at least +3. Savage Species is extremely conservative in its Level Adjustment Factors (11-14), saying that the gorilla avatar of nature's natural armor bonus, climb speed, and natural weapons are each worth LA +1 for a total LA of at least +3 without considering its other abilities. Obviously, that's way too high—even 3 levels of fighter would be better than just those abilities considering the loss of hp, skill points, base attack bonus and base saving throw increases and so on—, but that wacky estimate is something that should give the reader pause.
No Player's Handbook race has a natural armor bonus, climb speed, or natural weapons, for instance, and no race but the gnome has spell-like abilities (and the gnome's are one limited 1st-level spell and three 0-level spells), and no race has a class ability like wild empathy. If considering a gorilla avatar of nature for consumption beyond one's own campaign—you're planning to one day publish your own campaign setting, and you want it to reflect the game's incredibly conservative sense of balance—something will probably have to be cut.
If cuts are to be made, this writer would prune completely gorilla avatar of nature's spell-like abilities, remove its extraordinary ability wild empathy, and ditch the feat Brachiation (Complete Adventurer 106). Then he'd make its climb speed half the its base land speed and grant the creature a +1 natural armor bonus at level 1 that increases by +1 every 5 character levels or Hit Dice. Finally, he'd change the 2 nonstandard rend natural attacks to claws like the Monster Manual ape (268). (This last change would be made no matter what to prevent the creature from making rend attacks and claw attacks that were gained later.)
My opinion is that if PCs are starting at level 1 then the gorilla avatar of nature as it stands deserves LA +1. The level 1 gorilla avatar of nature water orc barbarian 1 PC will have Strength greater than her nongorilla peers so she'll deal more damage, a much higher AC than most so that she'll be hurt less often than her peers, mobility that'll exceed that of her peers (remember that the 3.5 revision made the spider climb spell a 2nd-level spell!), and even utility beyond that of some of her peers (a ranger that takes the alternative class feature spiritual connection (Complete Champion 50) at level 1 gives up the extraordinary ability wild empathy to gain, among other benefits, 3/day the spell-like ability speak with animals, for instance). Add to that her ability to make at least 3 attacks per round—that includes one with her longsword—and the ability to rage 1/day, that seems like a level 2 PC to me—probably on par with the capabilities of a barbarian 1/druid 1.
Were I to want to reduce this to what I thought might be LA +0, the easy way would be to eliminate the climb speed and the feat Brachiation completely, replacing those with a +4 racial bonus on Climb skill checks. Then I'd consider eliminating either the extraordinary ability wild empathy or the spell-like abilities. Finally, I'd probably reduce the natural armor bonus +2.
Then I'd playtest it and see how devastating I could make those 3 natural attacks. (For comparison, a DM using the Races of the Dragon Web enhancement may allow a PC kobold—with its LA +0—to have 3 natural attacks, but no other race I'm aware of does; each deals but 1d3 points of damage, by the way.) For example, a gorilla avatar of nature water orc barbarian 1 with the alternative class feature city brawler (Dragon #349 92), the alternative class feature spiritual totem (lion) (reskinned to gorilla) (CC 46), and the alternative rage whirling frenzy (UA 66) can charge a foe, pounce, and make a three unarmed strikes and three natural weapon attacks. When she does that, she may have—conservatively—a Strength score of 25 and perhaps even the feat Multiattack (MM 304).
Note that when the gorilla avatar of nature water orc barbarian 1 advances to level 2, she can take a level of rogue and with those attacks possibly deal +1d6 sneak attack damage. Alternative, she can keep things simple, take a level of warblade, take the 1st-level Iron Heart stance punishing stance [stance] (Tome of Battle 69), sacrifice some of her already significant AC, and just straight up deal +1d6 points of damage with all those attacks.
This is what would happen in my campaigns were I to make available the gorilla avatar of nature at level 1. At higher levels, this would be less of an issue in my campaigns as other folks will be able to catch up with class features, feats, magic items, and so on, but at low levels, if other equally strong avatars of nature are unavailable, other players will wonder, "Why am I not playing that character?" In your campaigns, though, maybe things are different, and players will look at that and say something like, "Hey, that gorilla avatar of nature would make a really flavorful shugenja." If that's the case, then you may have to go back to step one: "Is this appropriate for my campaign?"
About the Mechanics of Rebirth
Races of the Dragon on Mechanics of Rebirth (10) describes what a creature keeps and loses when it undergoes the transformation from what the creature was into a dragonborn of Bahamut. Using those as guidelines, below are some suggestions and things to keep in mind.
A dragonborn of Bahamut keeps the type and subtypes of its original and gains the subtype dragonblood. In a similar vein, for parity, I suggest that your homebrew avatars of nature gain a similar subtype called animalblood that sees the reborn creature still count as a member of its original race for the purpose of any effect or prerequisite that depends on race but also sees spells, effects, powers, and abilities that affect or target animals also affect the creature. The animalblood subtype qualifies a creature to use magic items normally only usable by animals, and it qualifies the creature to take feats that have the animal subtype as a prerequisite. The animal subtype also makes the creature subject to harmful effects that affect animals.
This, however, should be considered carefully. Some spells, for example, are balanced by the fact that they affect only animals (e.g. the 5th-level Sor/Wiz spell animal growth [trans] (PH 198), the 9th-level Drd spell nature's avatar [trans] (Spell Compendium 145)). Nonetheless, this advantage is likely counterbalanced by the many low-level offensive spells that can also only target animals. I urge making the subtype animalblood identifiable to some onlookers, not unlike the flaw Beastly (Dragon #329 95) that says, "Anyone observing your mannerisms who possesses the wild empathy class ability can recognize this flaw immediately."
(In fact, this might be what's meant by the question's entry of Type: Animal, but that entry isn't entirely clear to me. My hope is that, if nothing else, this section will help put some mechanics to that telegraphic entry.)
The creature keeps any languages it knows, but transformation into a dragonborn of Bahamut grants the creature the language Draconic. Similarly, I recommend that the creature that becomes an avatar of nature gain the ability to speak with animals of its picked avatar like a level 7 wizard's familiar's extraordinary ability speak with animals of its kind. (I assume that the original's bonus language Gora is, like, the language of sapient gorillas or something, which may be sufficient for your campaign but not be clear to outsiders.)
Keep in mind that the creature keeps its original race's ability modifiers and gains the ability modifiers of the creature it becomes. This means that any ability score modifiers supplied by the avatar of nature are in addition to the original. Thus, for example, a water orc (Unearthed Arcana 18) gorilla avatar of nature will see ability score modifiers of +6 Strength, +2 Constitution, −4 Intelligence, −2 Wisdom, and −2 Charisma. If the campaign allows the race, the GM should expect such a PC.
Finally, consider language that describes how the extraordinary ability wild empathy scales. As written, it doesn't when, typically, it does.
