I haven't noticed this app before, but it sort of reminds me of Tabletop a bit, but as a universal app rather than just for the iPad. I'm quite intrigued and I think I'll try it out soon.
Anyway, for now here's the app description ...
Gemma turns your iOS devices into a growing collection of music making apps that you can use simultaneously to create music at ease.
The app offers the ultimate collection of musical apps all right in your mobile device. Each of these musical apps are called Gems. New Gems are constantly being developed and added to the Gem Store. You can experiment with powerful touch instruments such as synthesizers, drum pads, ambient sound generators, sequencers, and samplers. There will be all kinds of Gems for all kinds of users. You can combine these freely to produce your own songs in the Ensemble mode – even if you’ve never played music before. Create music with your mobile device wherever you go.
- More instruments added
- Core MIDI support
- Accelerometer support
The app itself is free, but I'm expecting IAPs all over the place.