In my (admittedly limited) experience, SDL/SDL2 is more of a general-purpose library for dealing with different operating systems, not for abstracting graphics APIs. While it does include a graphics abstraction layer for doing simple 2D graphics, many people use it to have the OS set up a window, process, and whatever other housekeeping is needed, and instantiate and attach a graphics surface to that window. Then they communicate with that graphics surface directly, using the appropriate graphics API rather than SDL. I've done it with OpenGL, but my impression is that using Vulkan is very similar.
SDL_gui appears to sit on top of SDL/SDL2's 2D graphics abstraction to draw custom interactive UI elements. I presume it also grabs input through SDL and runs the whole show, just outputting a queue of events for your program to process.