I can butt in why not. The way I see this is you have asked a question that can't be answered by any serious person.
ZGEN or GTCB hum well good question, and anyone with a crystal might answer. The chances are good one of these investments will be better than the other for sure.
However: One may outpace the other and then fall behind, or die on the vein. They may play tag or one may be way ahead by next week and way behind next year.
So how can this be a serious question? You may be serious I'm not saying your not, but there can be no good advise.
I think you need to know a lot more before your guess can be serious. Where are each company in there development and delivery? What is the sales potential and what are the expenses? What is the PR and what other products are in the pipe line, how far away are they? I just scratched the surface. What is your investment time line? Are you willing to trade as time goes on or are you a buy and hold kind of guy? ~ And with all of this answered it turns into an educated guess.