I'm using the thunder app on Android and I'm somewhat new to all this. I can see content from all instances I've signed up for, or I can select an instance(lemmy.world or guncad) and only see content from there.

No. You add it as it's own instance.

You can also create an account in the guncad index forums. Then log into that instance through whatever app you're using. Most of the guys are moving there.