Pike, Long Spear. Problem Solved
With all those HAs. A piker is useless in situations like those. Hoplites and throwers with shields are much more useful.
Anyway, regarding the op. I don't think cavalry is overpowered, and I don't think it's any problem with the banner balance and having the majority of one class on one team. You can still if you play well and try to organize the team. It won't work all the time, but once you get an urban map then the GK team will have a real problem getting multipliers at all. I don't see anyone complaining about that though, when they're able to steamroll the other team who consists mostly of cavalry who's obviously a bit weaker on the ground.
A full GK stack will probably win most of the time on very open maps like ruins and the open deserts etc. but there's also urban/city maps in the rotation, don't forget. They pay for their efficiency on open maps by dedicating themselves to being cavalry, but it also brings that bad part, they'll have difficulties when the maps are less favourable.