Yes, that's something we have in mind.
The choice of Creole in 2013 was probably not the best choice we've done at that time. Actually we are even thinking about supporting several syntaxes (Creole/Markdown/AsciiDoc/...).
But right now, we can focus on gfm. However, this need a deep refactoring of our code, so do not expect this in the incoming weeks. We have to do this refactoring without breaking the actual support of Creole.
What would really help us is that you list the features you need from gfm.
We have started a doc page for this here http://alphadoc.plantuml.com/doc/markdown/en/gfm-support
Would you mind editing it with your need ? Thanks for your help !