How can I learn photo editing in GIMP more effectively?

The gimp docs, youtube and various gimp tutorial websites already mentioned in the comments are invaluable. I have found that I made the most progress when I tackled photos that I wanted to edit. Whether it was an event you attended, a holiday, a topic, subject or chosen destination that you had in mind (instagram, etc), start with some photos and experiment with "getting the most" out of them. For each photo try google searches like "gimp portrait tutorial", or "gimp separate out background", or "gimp restore old", etc. Gradually over time you will build up your repertoire of what you can do, and be able to selectively apply the techniques you've learnt depending on the situation. After a while, you will be able to drop gimp from your searches, and even use other tutorials but translate them for yourself into roughly equivalent gimp edits. (Admittedly not always obvious or possible.)

Have said all the above, a lot of photo editing to me seems to revolve around the following topics, so I would concentrate learning how to do the following in gimp:

1. Basic layer operations: add alpha channel for transparency, duplicate, create from visible, layer opacity, layer groups
2. Overall lighting (whole image/layer)
   • create a layer in overlay mode and use soft-brush black and white painting at different opacity levels
   • get to know the Curves dialog (Colors menu)
3. Selecting a part of an image to work on
   • using the different selection tools, adding/subtracting from selections, feathering
   • layer masks! takes some getting your head around if you haven't worked with them before (think stencil) but valuable, valuable, valuable! (I wish I'd started earlier getting to grips with them.)
4. Healing, cloning tools used with soft brushes at different opacities
5. Filter Enhance Unsharp, and Filter Blur Guassian-Blur
6. Layer (or Image) Transformations: Perspective tool, rotation tool, scale tool

One could go on (and other people will have different starting points) but with the above I find I can do a lot of what I want to do. Experiment much, have fun, trust what you think looks good.