Well.. Using GS1 is a bad way to script anything. It's so.. Tacky!
You're going to want setshape2 w,h,{tiletypes}. You can set it's tiletype to chair tiles. Unfortunately, I don't remember what the chair tiletype is. Anyway, it would look like this:
setshape2 2,2,{7,7,7,7};
For onwall checks, you'll want to use the boolean onwall2(x,y,w,h). For example:
if (!onwall2(x,y,2,2)) will check if anyway in a 2x2 rectangle of the NPC's x and y is blocking. If it returns false, it'll keep going.