Hi!

A statement first! GIMP is PERFECTLY fine for underground caverns, it is just to learn how to mould it into one. I agree with the others. The blurry texture messes with one's senses. That said, this is a good start.

This is an underground cavern map I did JUST in GIMP, just to show it is more than possible. random free underground cavern.
For this one, I used Mouse's very cool free texture.

Step one, paint in BLACK, or any solid colour, where your rock is going to be. On one layer. RIGHT click on layer up in right corner, and choose ALPHA To Select.
Two create another layer above that layer, drag an drop Mouse's texture in there, or any you want. But, one with nice random detail. (You can also, resize the texture to your canvas and copy and paste it in IF you don't want to risk random lines popping up, can happen.)
Three use that first layer you painted on. Still in alpha to select. Click on the SELECT tab and click on Grow Selection. Grow it by 2.
Throw in a Dark almost black brown into the selection. Remember, this layer is just below your texture layer.

Four create a layer below all your other layers and throw in a paper or ground texture.
Now shadows are EXTREMELY important. It gives the 3D look. PICK a side your imaginary light is coming from on the map. And start painting in shadows. Or use the Drop Shadow effect on the TOP layer or second to top layer. (Though be aware this effect has the very frustrating effect of increasing your canvas size. So best do the shadows by hand.)

Shadows and Highlights! Yays. Hehe
Okay for doing shadows, I would suggest creating a layer just below the second to top layer. (Definitely under your texture layer) (Use multiply mode)
Now with a SOLID BLACK or VERY DARK brown. Having picked from where the OVERALL light is coming from, just ignore that we are underground for a moment. Paint with ROUND brush where there would be shadows. DON"T TOUCH OPACITY. After you painted in your shadows, lower the ENTIRE layer's opacity to around 50% or higher. Just need it nice and dark.
Try that and see if you can figure it out. Don't be afraid to ask.

For the way I did the whole 3D slopes of the rock. Well, that is next lesson. It involves quite a bit of more advance techniques.

Hope this helped some!
Don't be afraid to experiment, and definitely don't be afraid to ponder on feedback.