GTK+ is a library for creating graphical user interfaces. The GTK+ library is also called the GIMP Toolkit.
GTK application need to run within the GUI environment (startx).
The GTK library is, well, kinda basic. It you're coming from a windows or mac environment then be prepared for a shock, gtk is no where near their league. The GTK documentation is also not great. The documentation exists, but its formal, there's quite a lot of depreciated things within it to trip you up / waste your time and there doesn't seem to be any really good resources we have found yet out there to tell you simply if you want to do this then here's the best way to do it and a block of working example code.
https://developer.gnome.org/gdk3/stable/ <<<The API reference manual