I have my own resource pack and I have added my own textures for this mod so it fits with my own pack.
But I can't add my own textures for the GUI. You know, when you open the chests.
When I open the JAR file and go to the textures folder there is no gui folder.
Just block, item and model

The GUI is managed by LibGUI. This avoid the need for a texture for the GUI. I am not sure how to fulfill your need in this case. My suggestion would be to check LibGUI wiki/documentation.

Thanks for the answer.
I guess I need to find an similar mod that do use textures for the GUI.