There's a few flame effect tutorials out there, but all the ones I've found are either badly outdated or too fiddly. This is how I set any object - including text - ablaze with the Gimp, in just six quick steps.
Done with Gimp 2.2.13, which I'll keep using for tutorials until more Linux distros start including 2.4 by default. If you've soldiered on to 2.4, the menus will be all switched around, but the effects should all still be there somewhere. Click images to get full-size shot.
1. Make your text. Plain white on black.
2. Flip the image with Image->Transform->Rotate 90 degrees CW. We're doing this because the next step doesn't go vertically yet.
3. Apply Filters->Distorts->Wind... The settings I have here are a good enough effect, but feel free to experiment.
4. Gaussian Blur with Filters->Blur->Gaussian... Now here, I have clicked the little link-chain icon next to the spin-boxes in the dialog to turn off the link between horizontal and vertical values, because I want them different. Horizontal is at 10, vertical is at 4. This is to make the flames streak more away from the text than side-to-side.
5. Flip it back counter-clockwise again. Select the "Incandescent" gradient.
6. Apply Filters->Colors->Map->Gradient Map. Poof! It is so.
Meh, of course, this is just a general method to keep in mind. A similar effect can be had with the built-in Script-fu extension "Glowing Hot" logo. But this tutorial works for any object, not just text. You can get plain old fire from a solid line...
...or combine it with another text effect, putting the flames behind it.