I was wondering if the arnica gel would help with my fibromyalgia pain?

Yes, it is licensed for muscular aches, pains and stiffness, but I'd say that you need to take a fairly comprehensive approach if you are going to control this condition.

Cutting out tea and coffee and increasing magnesium-rich plant-based foods should help reduce pain perception and headaches, and improve sleep quality.

Some people have used Hypericum (St.Johns Wort), not because they are emotionally low (although fibromyalgia makes plenty of people feel that way) but because they have found it to help pain. That's why doctors often prescribe antidepressants in cases of fibromyalgia.