I don't see how people can think Simon isn't GAR. At all.
He literally only cared about other people, and didn't care what happened to himself. He went into battle and fought for the entire fucking universe, against the Anti-Spirals.
Look at when he was in jail and he saw the Arc-Gurren lift off. He smiled because he thought everyone was safe. He didn't care if he was left to burn and die on Earth as long as everyone else was fine.
After saving the entire fucking universe, what did he end up with? The only two people he'd ever loved, his best friend, and his wife, were dead. Not only that, but he was so powerful that if he continued fighting he would cause the Spiral Nemesis. So he couldn't fight either. He knew he wasn't a good leader, and that he was just a fighter, fighting for others, and that Rossiu would be a beater leader for Earth.
He'd lost everything that mattered to him, the people he loved and his ability to fight for others. So he left. And he spent the rest of his life, from the age of 21, alone.
In other words, he literally fought for everything, gained nothing, and lost everything one could lose in the end, and after all that, he still smiled and was hopeful for the futures of others, as he travelled the Earth to see the people he saved.