Gimp does offer something similar, I use it to draw my characters and all my Fan Art, etc. Here's a useful tip to drawing colour and then shading onto lineart, it should work for this as well.
1. Select Layers Tab
2. Create a new Translucent Layer under your current background
3. Create a white layer below that.
4. Click on Translucent Layer and click the little drop down menu (still on the layer tab) It should say mode beside the drop down menu, and click Multiply.
5. Colour on top of your translucent Layer, if you go over the lines you can rub it out without rubbing out the actual original art.
6. Highlight the transparent layer when done and click on the little check box beside the little icon of the checkered box on the layers tab. This makes this layer fully transparent so you can shade it.
7. Click air brush and set the opacity to around 21, choose a small size 7 brush and draw on the white layer.
Congratz you have learned how to colour and shade in Gimp.