Hello PlausibleJosh, welcome to the Guild!
There are a lot of brushes and other tutorials for GIMP, some for older versions, but all should be close enough that you can work out the minor differences. For example, the brushes have a shape (circle, etc), a size (09, etc), and a hardness (25, etc). Just find the place in GIMP where you choose a brush, set it to a circle, size 9, hardness 100. Draw lines that look like mountains
The fadeout is to fade the lines at the bottom where they join the flat land.
Using premade brushes in the shape of mountains is easier. Just get the brushes you want in an .abr file. That's photoshop, but I believe GIMP can use them as well. Then select one of the mountain brushes (the shape), size it to whatever size you want, hardness 100, and click it to place the mountain.