Here is Pasis's Tutorial for Photoshop and most of it can transfer to GIMP minus layer effects which he mentions.
[Award Winner] Rising up the mountains in Photoshop
Here is Ascension Excellent "Atlas Style" Tutorial for again Photoshop but again alot of it is similar to GIMP.
[Award Winner] Making a continent in photoshop
And from the Grandfather of GIMP mapping RobA's Tutorial for GIMP.
[Award Winner] Using GIMP to Create an Artistic Regional RPG Map
That should get you started. I use GIMP Exclusivly (with Wilbur) and these three tutorials taught the basics of what I have learned over the past year or so in mapping... There is an entire Tutorial section in the forums (its at the bottom) to exploit. Dont forget to experiment too!
EDIT-> Start with RobA tutorial as its in GIMP and will help with the "how do i use this thing" then go the Photoshop Tutorials so you know what tool/command will do the same thing in GIMP as the one listed for PS.