Another way to create a clipping mask is by holding down alt, and then click in between the layers you want to clip, your cursor shall change. If that doesn't work, your version of photoshop may not use clipping masks.
You mean when you change the color temporarily or when you use the gradient? You can skip the step of when you color over the white, it shouldn't effect anything.
Hm :/ I couldn't tell you what went wrong, but maybe your lines weren't closed, you forgot to make a clipping mask on one of the layers, you forgot to hit select inverse at one time, or something else.
Idk, maybe make just a small quick picture just to test out the method, and practice a bit until you get the hang of it c:
If Gimp has a gradient tool then it should, because it is possible to do it without the clipping mask, it's just more difficult.
Just after you put down all your gradients and colors and stuff, everything will be out of the lines, which a clipping mask prevents from happenning. An easy way to clean things up would be to take the selection tool/magic wand tool, if Gimp has one, select outside of the line art, go to the layer that needs to be cleaned up, and press delete. It is possible to work around, just more difficult c:
wow, that is some really strict internet provider you have if you want, i can make it 10 mb parts each and send it to u in the email or upload it somewhere "if u have a good file sharing site let me know" .. the internet is all about freedom, thats how it should be >.>