I've been trying to test some theories for an hour almost now.. Found nothing, but something that i did notice is that, the Time Vex Gates can be activated when Atheon is dead without anyone standing on it, .. Idk.. Maybe try and keep someone behind the door to the gatekeeper and jump on the time vex gate after atheon is dead, and try and activate all 3 vex gates? And maybe thats your secret ? Lol