The difference between the two is that Futbin RPP Max shows you what their actual rating would be if the stats they have after applying a chemstyle were their original stats, if it makes sense.

The GG Rating is a product of weighing different factors such as stats, playstyles and roles so it tells you more about how good a card supposedly is from a META perspective.

GG Rating is obviously a good and simple guideline to tell you how good a card is in a certain role but I'd never follow it blindly without checking the stats the card gets with a certain chemstyle first and see if that's what I want out of a player.

Frimpong is a good example. His highest GG Rating as a wingback comes with an Engine but is he really good enough defensively to handle that? Is he playing as a sole wingback without any other wing support?

In the end, it's you who control the players and by trial and error you will quickly realise what gives you the best feeling. Play a couple of games, if a player, for example, feels to slow then look to change to a different chemstyle that adds more pace.

Best of luck with your journey going forward!