Thanks! Glad you are learning with it! That was the original objective of the library! Yeah, I learn about Tsoding recently and saw some of the videos! Really nice!
About the issue with raygui you comment, it seems related to calling some raygui function before raylib InitWindow(), an OpenGL context is required to call rlSetTexture()... ok, I see you got to a similar conclusion, yeah, it could be that raygui as a separate library can not access the raygui library OpenGL context.
My recommendation is to compile raygui together with raylib, afair, the default raylib Makefile includes an option to add ragui module into raylib on compilation.
My recommendation is to compile raygui together with raylib, afair, the default raylib Makefile includes an option to add ragui module into raylib on compilation.