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'm... not sure how that would work, but I guess it's magic so it can work in any way the DM wants to 

I mean, I know Mystra is a god so could possibly do a lot of things, but severing someone completely from the Weave? Did this happen before? Is it even possible? From my understanding the Weave is one of the fabrics of the world, anyone with enough talent or knowledge can access it and make use of it, like any other element out there.