What issues did you have with shaders? Unity doesn't always make documentation easy to browse, especially when it comes to shaders, but outside of shadertoy its

What issues did you have with shaders? Unity doesn't always make documentation easy to browse, especially when it comes to shaders, but outside of shadertoy its one of the best environments to just start writing shader code with limited setup. If you don't understand Unity (like what a game object or component or material is) it will be challenging to drive into writing shader code first though. There is a MOOC on Udacity that is introductory, but could be good for you if you are a beginner. A lot of the deep information about graphics is in vendor documentation and GDC or siggraph presentations. Some of it is in blogs or books. If by GPU programming you're thinking more compute focused than graphics, reading Nvidia's CUDA documentation and tutorials could be a good start.