...make realistic coastlines and topography in Gimp?

[Jaxilon]

There are a number of great GIMP tutorials in the Tutorial section of the site. I would recommend going through a couple of those so you learn some of the techniques for GIMP. I know there is a coastal tutorial and I'm pretty sure I remember a mountains one. At least you have some PS background. When I joined here I didn't have any digital background. Gimp was my first such application and I've only done a couple of the tutorials but have learned tons.

[Bambi]

Thanks, there are heaps of great tutorials, think I'll just have to keep going through them and trying different techniques till I find what works best

[Solomiranthius]

I'm a GIMP newbie too and found these tutorials really helpful, although it might not be quite the style you're going for?

RobA's "Using GIMP to Create an Artistic Regional RPG Map" and "Simple Mountains in GIMP"
Arsheesh's "Making Photo-realistic Trees in GIMP: A Mini-Tut"