Yarr! Welcome to me ship, landlubber!
Hey and welcome to Gimper! First of all, I don't think we have got many painters around here (I've got a tablet but really I'm not too good at drawing anything), so we probably can't help you at developing new drawing techniques. On the other hand, you may become our #1 painting pro if you stay

However, everything that is Gimp related will be answered here, no matter how "nooby" the questions might seem.
Here are some tips I can give you:
- If you're still a beginner in painting mangas, read a lot of tutorials. deviantArt.com, for example, is a great site to learn about painting techniques and methods.
- Download and install Gimp Paint Studio. I've been using it ever since I bought my tablet and it really adds a lot of useful things to the basic Gimp 2.6, like tons of brushes, patterns, color gradients (for example many skin-color gradients, tree colors etc.). If you're really not feeling any comfortable with Gimp at all, try MyPaint, another freeware program especially made for painting.
- Show us some of your artwork if you want to. Although we're not the best painters the interwebz has to offer, we can probably give you some hints when it comes to things like colors, shadows, perspective etc.