You need the correct animation frames or frame for sitting. There should be a script available by default that allows you to step through animations frame by frame called, ai_viewanimations. It allows you to see all the available anims for characters. Find the sitting anim, make a note of the anim frames, then set your anim to match those.
Pretty sure these will work for the default GG characters. But keep in mind my advice to find other objects correct frames.
SetAnimationFrames(3100,3830)
That code only sets the range up you still need to play animation after. Also I think it is a talking sit down anim as I used it in this.
Edit - After seeing the start of my old video it seems he is standing and talking, still finding the sitting anim should not be hard now.
SPECS: Ryzen 1700 CPU. Nvidia 970GTX. 16 Gig Memory. Win 10.