There are minimal images of M1025's fitted with GPK's. You can probably do it and be ok. I've never seen, in person or in photographs, an American M1025 fitted with GPK but it doesn't mean it didn't happen somewhere.
Here is an image from 'Humvee's Around The World' that shows Iraqi M1025's fitted with Armor Survivability Kits (ASK) and fitted with GPK's.
Here is a Polish M1025A2 with GPK