Stormclouds gathered in Gale's eyes, but nothing that carried anger - just sadness, the morose grey of a tombstone, a whole row of them. "I wanted to - but I thought you hated me," he admitted, and something caught in his voice despite his best efforts to conceal that emotion. It was never anything he was great at (emotion, that is) and that may be why Katniss had chosen to be with Peeta - who was always so intelligent about the way people felt. Sensitive to it. Gale just wasn't - he barely understood how he felt half the time, much less someone else.
"I shouldn't have made assumptions about it." At the very least, he knew that much. "I'm sorry about everything. That happened between us." Liberating Panem was important to him, but so was she - he didn't want that to get swept up in waves and dissolved, not again.
He may have believed in their new government, somewhat, but he'd always be wary - Gale knew what power did to people. If he stayed in Two, he could be a strong, sturdy presence to remind everyone what this newfound freedom cost, so they didn't regress. If he stayed, if he was loud enough - and someone like him was loud enough.