7+ would be fine. I was running MIDI Guitar with a 6 (with a cracked screen LOL) for quite a while.
The GP-10 wouldn't be involved at all. You'd just plug the cable into the Helix LT's USB jack, and the other end into the phone, and Bob's your uncle. Jam Origin now supports AUv3 plugins (I'm really into the aforementioned SoundFont Pro at the moment), so you just save presets with your plugins loaded in place, and use PC messages from the Helix to select them. (I recommend having a first preset that's completely silent.)