The problem seems to be getting the food through the gg to the plec? Our gg eats everything but the plec still gets fed as there are hiding areas for it under the wood where veggie food get missed, not eaten etc. Try getting food into a place where the gg can't get at then the plec can eat it there.
I've also heard of people losing discus to plecs that suck them to death
If it doesnt stop, you will have a stressed and eventually dead GG. Sometimes when they start doing that, they don't stop... It's too easy a food for them, and this food is like candy to them.