what does GBL stand for? something bakugan league?
I've never seen or heard of any GBL cards...

Oh wait... you mean Series 2+...
I know in series 2 and beyond BAP is Bakugan Promo
and CN is obviously a Cartoon Network Exclusive card ( like the one you get in the vol. 1 dvd... though I think they changed that for the vol.2 card)
And obviously standard on any card like before is the Card type.. GA - Gate Card, or AB - Ability Card.
But you've got me on GBL, I've never even heard or considered that.
And on closer inspection of my series 1 cards there doesn't seem to always have to be a B on the end for the new artwork; it is definitely the lowercase a or b at the end of the number that designates it.
(For clarification, there are some new artwork card without the B, but no old artwork with the B.)