GW was designed to go right on top of MP, so you don't have anything to worry about.
That being said, I find that all sealants have a little bit more longevity when used after a good AIO cleaning, but that's just my personal preference. Like the others already said, both will work great.
I don't know if you've used GW already or not, but here's a tip. Be sure to apply it as thin as possible. Almost to the point where it's hard to see if you even have anything on the surface otherwise it can be a bit troublesome to remove.
Also, treat it like SG. Either remove it immediately or let it dry for at least 30 minutes before removing. It doesn't like to come off during that inbetween curing stage.