I am trying to find a good answer to what the native API is for doing GUI applications in linux.

From what i have gathered X11/xlib.h contains the most native API for GUI applications in linux, then other libraries like GTK+ and Qt is built on top of this API.

My question is if my statement is correct and also if there is an even more basic way of doing GUI other than xlib?

There is one more level below Xlib. Newest versions of Xlib library use XCB to perform its work: http://xcb.freedesktop.org/
Read about differences between Xlib and XCB here: http://www.x.org/wiki/guide/xlib-and-xcb/

nxsy have created HH platfrom layer on Linux using XCB here: https://forums.handmadehero.org/i...p/forum?view=topic&catid=4&id=151

BTW, xcb is autogenerated from protocol definitions.

Also, it only applies to X. If you use wayland or mir, you'll have some other protocol/library to use. There is also hardware-accelerated OpenGL which can be used to draw things on screen.