You don't have to read the documentation "cover-to-cover" to learn from it. Just skim over the main classes (Window, Image, etc.) and look at the included examples. When you have a general idea of how things are done, start making something. If there's something you want to do, but you don't know how, take a look in the docs. There's really not that much to Gosu (on the surface) and it's really well organized and logical. You should be able to pick up the basics very quickly with just a bit of practice.
Also, don't forget to use the search engine here on the forum. Most likely, your question has already been asked and answered at least once.
By jlnr (dev)
Date 2013-05-17 04:58
There is no definitive list of concepts :) If you want to write a jigsaw puzzle game, the code will look completely different from a Super Mario clone. Gosu makes almost no assumptions about what you want to do.
I would suggest that you download a few smaller games from the Showcase and mod them a little, and get a feeling for which code style you prefer.