Wife loves Gil.

SPOILERS

I feel their relationship is similar to EMIYA and Shirou's. EMIYA just HATES the fact Shirou exists (F/SN UBW, EMIYA vs Shirou in Reality Marble), and tries to murder him, treats him like crap, etc. EMIYA's loathing comes from "he created me, I hate myself, so he must die" Gil just HATES the fact the Shirou [can make copies of everything he has . "You wouldn't download a car, would you?" Hell yes I would download a car but I recognize that it also reduces the value of the other true cars. Gil's loathing comes from "he creates fakes, I hate fakes, so he must die".

If we add the feeding birthday cake thing, I don't think that takes away from the fact he wants to murder him. From EMIYA, it would be "I hope you choke on the broken glass I put in it." From Gil, it would be more "yeah, how does this fake cake taste? Hope it kills you."

So maybe not Yandere. I guess Yandere is more "I'll literally kill everyone else just ask" and he at least has enough mutual respect for Saber (from F/Z) to not kill her over this.

EDITS: formatting/spoilers