On the labels of some mediums and gels it says to abrade the surface for better adhesion. My question in particular, I am coating a surface with 3 coats of GAC 100 to combat SID. My next step is to gesso. Do I need to abrade the GAC 100 surface (which is very glossy) before I apply gesso?
No.

I read somewhere that acrylic loves to stick to acrylic - so when do I abrade and when do I not have to abrade?
Abrade before you apply the acrylics, and do so only when the substrate isn't absorbent. IE Plastic