I will echo what breakingbenjamin stated and add to it.
I don't like any of these names personally because to a new player they don't gel well.
Academy I think is the worst name because to me Academy refers to school or learning or practice and between 'Campaign' and 'Challenge' and 'Practice', adding a "Academy" would just overly confuse new players.
Foundation would be the best name in my opinion. You already have Foundation Cards which are free to earn for players and so continuing on with that same theme of "Free2Play" being Foundation would make the most sense to me.
However given the 6 options, Gateway is going to get my vote because it is a Gateway for new players into this whole realm of Splinterlands. Like a Gateway Drug, give them a taste to hook them!