Turn on the Gemgun without putting it on your body. That may seem like a minor thing, but it’s important for better control. 
Rest the Gemgun on your body. Don’t add any pressure. Just let it float.
Glide the Gemgun along with the muscle. Move slowly - about an inch a second or slower. 
If you hit a knot or an area of tension, let the Gemgun rest on that spot, without adding additional pressure for a few moments before moving on. 
Breathe: nice long inhales and exhales. And feel the relief.