<ProjectName> -> right click -> Properties -> C/C++ General -> Paths & Symbols. Then in the right pane the Tab "Includes" ... (check two times, one for C, one for C++ language)
Take a look at this video, it demonstrates a secure way to integrate tGFX 4.13 in CubeIDE 1.2.1. Works with minor changes also for 1.3 (One time it asks for updating to a newer version, just click "Yes" there...). This is the way, how i begin my projects at the moment. Beware: It´s just a minimal hardware setup, so additional functionalities have to be added with cubeMX... But anyway it´s a very good starting point :)