Here are two tuts that I believe any gimpuser in particular (and any one in general) should have a go through soonest.

Layer Masks by jfrazierjr
http://www.cartographersguild.com/showthread.php?t=3143

A great guide to using layers to create effects and stuff

and

Using GIMP to Create an Artistic Regional RPG Map by Gimp Master RobA
http://www.cartographersguild.com/showthread.php?t=1142

It might look intimidating for a new Gimp user, but there are sooooo many things to learn about Gimp in this tut. It is well written I think and plenty of examples, and easy to follow. Read the whole thread if you have questions because many before you have asked and answers are aplenty. Ask in the thread if there are any questions after that and you will be pleasantly surprised how quick the responses come.

Enjoy your stay