It's a good ruling
Either your GM didn't know the RAW ruling at the time, in which case he came up with a ruling off the bat that he thought would be balanced, coherent and believable, or he did know the RAW answer and chose to ignore it, as I would have.
The problem I have with the RAW ruling is that it clashes with my view of the scene. A strong NPC is ...
It's definitely against the rules, but not unreasonable
The scenario as presented is fairly reasonable, but the ruling isn't RAW, and if this irritated you it's completely justified.
Plainly put, forced movement breaks grapples, and it doesn't care which creature is moved.
PHB pg. 290 under conditions, Grappled:
The condition also ends if an effect ...
Is it reasonable to make Water Whip a tug of war, requiring a Strength saving throw against my characters' Ki Save DC, in situations where a target is being restrained?
The specific ruling is probably not RAW, but could be entirely reasonable IMO. There is good precedence in the case that a target is only grappled, then any forced movement should still take ...
The 3rd-level druid spell entangling staff [trans] (Spell Compendium 83), in part, says
If your grapple check succeeds, your quarterstaff’s vines constrict your foe, dealing 2d6 points of damage (you can choose to deal nonlethal damage instead of normal damage if you wish). You then have two choices:
Release: You release your opponent from the ...
"When you hit a creature with an unarmed strike or an improvised weapon on your turn, you can use a bonus action to attempt to grapple the target. " - Tavern Brawler
If you were to use this feat, you would grapple as a bonus action after making an attack action and then you could use your main action to pin.
Otherwise, not possible as stated above.
If the kraken wants to bite a creature grappled by its tentacle then eithier it needs to move its mouth to the creature, which it can do through moving, or move the creature to its mouth, which it can also do through moving as follows:
Moving a Grappled Creature: When you move, you can drag or carry the Grappled creature with you, but your speed is ...
This doesn't work
The attack action is only able to perform the grapple:
...you can use the Attack action to make a special melee attack, a grapple. If you're able to make multiple attacks with the Attack action, this attack replaces one of them.
The pin ability granted by the Grappler feat is a different action altogether:
You can use your action to ...
From page 176 of the Dungeons the Dragoning 1.6 core book:
At the first level of any sword school, a martial adept gains access to the Universal
Advantages and Restrictions, and may use Standard Attack actions to build Special Attacks. These represent the basic training and universal abilities that all adepts can access.
So every sword school allows ...
This works, but probably not the way that you want it to.
Let's start by looking at the relevant portion of Double Slice:
Requirements You are wielding two melee weapons, each in a different hand.
You lash out at your foe with both weapons. Make two
Strikes, one with each of your two melee weapons, each using your
current multiple attack ...
I've encountered a situation like this while GMing for a group of friends. I ruled that freehand used for the grapple is occupied as it needs to be used to continue hold onto the creature. To make the attack with the gauntlet would require you to let go therefore releasing the grapple but according to the core rules the grapple can be released as a free ...