I like the general composition, here's some things:

Learn about GIMP's layer masks, they are really handy when you want non-destructive transparency in a layer.

The mountains are pretty strong everywhere, you don't need to get rid of them, but you could try to create some variations in elevation at the edges: layer masks can help with that, because a translucent relief mountain looks lower than an opaque one.

A simple way to create a white outline around text in GIMP:

There's a tool called Select By Color (fourth button), use that to select text that is black (on a specific layer), this gives you an outline selection of that text. In the menu, use Select > Grow to grow that selection outwards by X pixels (1 or 2 should be fine). A quick way to fill a selection or layer with colour is to just drag and drop from from the color indicator into the selection, and bob's your uncle.