Maybe you should explain more throughly what it was you wanted to do. Sounds like to me from your reply you just want something you can put an image on already sized for you and you just print, cut out, and paste to a bottle cap???
If that is so only you can do that by measuring your bottle cap you want to place an image on. As an example, by tracing on a piece of paper the outer edge of your bottle cap, then scanning into your pc, opening bottle cap image into gimp, and play with gimp until you get the result you want and keep it as a template.
Then you can add images as a new layer to your bottle cap template. Just make sure you have your dockable layer tab

open so you can see all your layers and resize with available gimp tools; such as using the

tool to resize the image you want to place on it.
If this does not help please explain yourself better and if possible give the group an idea of what you are striving for. The regular GIMPers on here are a great resource and very helpful. If they can see what you are trying to do they come up with loads of ideas to do things.
Liana