i was looking though the steam sale and saw divinity original sin 2 and decided to click and look and then i saw that it has a DLC "Companion: Sir Lora The Squirrel". why isn't this DLC on GOG?

The squirrel is automatically in the GOG version.
It would be better if it wasn't and you had the choice to install it or not, because you can't kill it.

squirrel companion dlc is for the original game. The DLC was a preorder bonus i think. The DE edition already has Sir Lora DLC included. It isn't much just some conversation over the course of the game not really a DLC just a companion you can talk to.