Hi @francoisgrogor, I'm Semoju (Creator of gemionic and rubyonic) Thanks for the feedback!
To answer your questions:
Support We're regularly updating. Gemionic just got a major update today with a brand new design. You'll be able to add the generic ionic components in it and create new screens with minimum fuss. You can see the difference at http://market.ionic.io/themes/audacitus (APP ID f32151ec)
Converting your existing application If you can integrate your app using the any of the starter themes, its possible to integrate into other themes. Depending on your ability to replace the static content with your bindings and controllers while familiarizing yourself with the states and extra markup, the time involved boils down to your pace. But Gemionic is based on the sidemenu starter template, and doesn't stray from that file structure. So it should be easier.
We'll be adding more updates based on feedback, so if you don't see something that you would want, let us know.