Well need to see more of his story but I think Gale is lying.
It doesn't make much sense that he's gaining power/levels again especially if mystra is ignoring him.
I  have a feeling that he's absorbing magic items in order to power himself back up after mystra removed 
 his connection to the weave .
I agree. But we know that all the origin characters have a redemption and damnation path.
But "does Gale make sense as a sorcerer?" isn't a question that can answered until his questline has been completed, and the answer may be different if he is the protagonist or a sidekick.
Personally, I plan on leaving everyone with their original classes on first playthrough, then maybe changing things up once I know how things pan out.