There really is no perfect gearbox out there, but either of those is pretty decent, but each with there own flaws. G&P is a really popular brand for people who upgrade like crazy since they are almost 100% TM compatible, however, it is hard to say for sure whether either or any gearbox for that matter will hold up on an 11.1 for very long. It isn't really a matter of "if" it's more of a question of "how long?". G&P are known to sometimes have problems with the gear axles snapping off and for much of the rest of the parts, there are better options available if you are willing to upgrade a bit. The same can be said for the other, which is definitely without its flaws (auto shim gears, etc) Aside from issues with those particular gearboxes, an 11.1 will cause wear on the trigger contacts if not used with a mosfet, and deans would be highly reccomended. Many people run their stock acm etc. gearboxes on 11.1s with no problems at all, and other boxes sometimes eat something right away, so QC is a big factor here.
I really don't see why you don't just use the stock gearbox and upgrade a bit. Just search around on here or somewhere such as airsoft mechanics for some decent parts and techniques. If you plan on running an 11.1 you probably should be comfortable with opening up your gearbox because despite what people say, there is no perfect 100% reliable drop in or stock gearbox for an 11.1 lipo.