To be honest, the coastline is right off your map, with a bit of a ripple effect to break up the straighter and smoothly curved edges. I just used a magic wand tool to select the ocean areas, removed the text from the selection, then filled it on it's own layer.

After that I used a combination of the masking tips presented by craggles, and layer styles as per Pasis' "Raising up mountains" tutorial - but those are pretty photoshop specific. From what I understand it's a tad difficult translating photoshop tutorials into gimp, due to the differences between how they each handle layers.. but we have some high ranking Gimp experts on this site, so you should find plenty of help in that regard.

Really, the thing to remember is that things aren't actually as difficult as they seem at first. Don't be afraid to experiment a bit - but wait until you understand what each step is doing before you start tweaking it around As you're just starting with gimp (as with any tool) I recommend playing around with the different tools for awhile, get a feel for how some of them work. Then break into your tutorial, and follow the steps exactly. Then go back and start playing with the different settings and see what happens. As you do that, you'll start getting more of a feel for how different setting affect what you're doing. Then before you know it, you'll be teaching us new tricks! (don't laugh, I've seen it happen)