UV Map in GIMP?

I can't seem to find a way to layer a UV Map on a template in GIMP?

Is there a secret? Can I join the secret UV Map in GIMP club?

I'm not sure what you mean. Isn't the template the UV map?

Anyway, you want to create a new layer over top of the template. The "Layer Fill Type" of the layer you create will depend how you want to work, but any of the the types can work, as follows.

If you create a transparent layer everything underneath (like the template) will be visible and your painting will be like drawing on an overhead transparency. It will look like you are drawing directly on the template, but since you are drawing on a separate layer you can undo/redo on that layer to your heart's content and not destroy the template, and then later hide the template. In this mode, the transparency control for the transparent layer will determine the opacity of what you are drawing. Use this control to peek under your drawing without hiding it entirely. You might even leave it semi-transparent while you work so you can see the template even through what you are drawing, and then adjust it to full opacity when you are done, or to see your progress.

If you create a white, background color or foreground color layer your termplate will be hidden. Use the opacity control on the new layer to peek underneath. Leave it semi-transparent while you paint to see the template, and adjust to full opacity to check your work without the template. Much like above, except that you start with a piece of paper rather than a transparency. Use the eraser to make portions of your drawing transparent, as when you want to "erase" the sleeve portion to create a short-sleeve shirt.

In any of these modes, any spot you leave the template showing, the layer below your article of clothing (or the skin) will show when the avatar is wearing your item.

In any case, when you are done (or when you want to check what your creation looks like without the template), adjust the transparency controll to full opacity and hide the template layer.

You'll quickly find use for multiple layers. Basically, anything you want to draw without destroying what's already there will go on a new layer.