Templates can be saved in any format, more or less, and are used for things that you plan on making... a LOT of. Pretty much, you just make the base of whatever it is you want to make a template for. Save it as a JPG, PNG, GIF, etc, so you can easily access it in the future. Pretty sure PSD files don't open in GIMP, but the rest of the image files I mentioned are, so you can do that.
To use it, just open up the file and make a new layer so you can do whatever you want. I would also recommend duplicating the template layer, just so you don't mess it up. It's more of a safety measure, so you don't have to do that. Layers are your friend, since it can save you a lot of headaches in the future.
I hope what I said makes sense and it's not all convoluted.... XD