I am going with my own theory..
Future GARlock, the apprentice of Simon.
Gimmy looks like a cool candidate, but with different VA and different hair color, kinda blew it away... and yeah, is this guy mid 30s!?
GARlock (real name currently unknown) is the kid Simon was teaching how to use a drill in the epilogue. At some point later the kid found a core drill while digging around an old ruin. Simon quickly becomes his master and teach him the spirit of the true man along with how to use Spiral Energy. Before he died GARly and meet with Nia, he entrusted the kid to Boota. He of course gave Boota his spiral power before he died, Boota transform to be Shouta Aide again.
The kid (currently unnamed) and Boota travel together as comrade all over the world gaining new allies and reviving fallen technology such as Dai-Gurren and re-form the team under the name of Gurren-dan. He gets to meet the old characters such as Yoko, Reitei etc. His journey will be in manga version, where he discovered the true meaning of the bible that Rossui used to hold and the true way to stop Spiral Nemesis. We will get to see him journey across Universe visiting several planets as the truth unfold.
Later he reunites with Chou Ginga Gurren and quickly becomes its leader known as GARlock. His story is yet to be told...
Above is my theory; let's look at other theory:
Popular Theory 1: Its all but Boota's dream - from my watch its pretty clear to me that Boota was fully aware whats happening when Anti-Spiral cast Sleepga on him. Besides.. whats the point of having some random scene be a prologue.
Popular Theory 2: Its Gimmy. Putting the fact that I don't think Gimmy has big enough balls aside, the VA voice (in credit list) and the hair color is totally different. There is also a possibility that Gimmy might use Science to stop himself thus cut off all the possibility of changing VA by aging. If you calculate it real quick Gimmy was probably ~14 before the time skip, after that he must be 34!! He still looks really young for some reason.