Just installed Guitarix on Gemini in Debian....
During install, it asked me about jackd, but said if you say Yes to permission request, it will devote all resources to it, which may cause device lock-up problems. So, think that is what happened to you, Gemini doesn't have enough power to devote all the resources to it. So, if you can figure out how to reverse this, you might be OK.
HTH,
Mark