Welcome @jack_bean!
Forgive the following Level One Tech Support question: Did you open/started a new image? The G’MIC-qt plugin remains ghosted and unresponsive until there is at least one image layer loaded. I know, a very basic thing, but I’ve Been There and Done That: thinking G’MIC is broken because I didn’t load an image first.
Also, as @Claes suggests: some details would help. A small narrative of what you tried, what you expected and what you got would help. Screen shots too. Some environmental information: your computer, your operating system, how much RAM you have. We can sherlock for you, but need some clues. Thanks! and Welcome!