All limitations were given by the spell's description:
This spell functions like greater polymorph, except that it changes one object or creature into another.
So, with the exceptions of everything after that, refer to the text on Greater Polymorph, which functions like Polymorph.
Greater Polymorph then says that if you assume the form of an animal, it works like Beast Shape IV. If you assume the form of a dragon, it works like Form of the Dragon I. If you assume the form of an elemental, it works like Elemental Body III. If you assume the form of a plant, it works like Plant Shape II.
The stats bonuses are all given by each of those spells, the difference is that Greater Polymorph will allow you to become creatures or objects of sizes different from those allowed by those spells, so size modifiers might apply. Also, the duration of this effect potentially much better.
So, for instance, you decide to turn into a Huge Mammoth, you refer to Beast Shape IV and see the benefits you gain from it, such as:
Huge animal: If the form you take is that of a Huge animal, you gain a +6 size bonus to your Strength, a -4 penalty to your Dexterity, and a +6 natural armor bonus.
If the form you assume has any of the following abilities, you gain the listed ability: burrow 60 feet, climb 90 feet, fly 120 feet (good maneuverability), swim 120 feet, blindsense 60 feet, darkvision 90 feet, low-light vision, scent, tremorsense 60 feet, breath weapon, constrict, ferocity, grab, jet, poison, pounce, rake, rend, roar, spikes, trample, trip, and web. If the creature has immunity or resistance to any elements, you gain resistance 20 to those elements. If the creature has vulnerability to an element, you gain that vulnerability.
Everything else is covered by the text on Greater Polymorph. This transformation's duration is much higher than 1 minute/level from Beast Shape IV, probably going to last permanently (5 from same kingdom, 2 from same class, 2 from same or lower intelligence, total 9 or permanent duration).
Alternativelly, if you decide to turn yourself into a Huge Mammoth Statue (not an animated statue), the duration factor would be only 4 (2 same int or lower, 2 mammals) and last up to 3 hours.