Does anybody know of any good tutorials for shading (with black, white, and gray) in GIMP, or have any good tips on how to do it?

Mushroomcomix
(with black, white, and gray)

If you're planning an actual color comic then do not use those for shading, that's the amatuer colorists most common and ugly flaw(next to dodge/burn tool) If you're doing black/white comic then ignore this :)

Shading is more about understanding light sources, color theory, and layers. they have lots of articles on them if you use a search engine.

Shading in gimp just make a new layer and setting it to multiply. Adjust the opacity if it's too dark. Figuring out the right brushes to use and light sources will come with practice. I'd recommend a tablet since shading by mouse is time consuming and hard by comparison-- but untill you get photoshop or another higher quality program your pens pressure sensitivity is near useless.

Remember layers, layers, and more layers.

i only use 1 layer for colors/ shades/ and highlights

It's worth noting that although Gimp comes with a very limited set of brushes, you can edit your own, or create your own from practically any image and import them. You'll also find that most free photoshop brush downloads are now compatible with Gimp. I think the Gimp manual is pretty good on how to do this, but I can remember how to do it if you need to give me a shout.

Also, you can get a dotty screentone effect, with the newsprint filter, if you're after that kind of look.

Hey thanks everybody!