GTK+ started as the Gimp Toolkit, GTK. Gimp pioneered the cross platform library for graphics. The Gnome group grabbed GTK, split GTK into smaller components, and added stuff to make GTK+. You can use GTK+ in most operating systems with most languages.
Qt and GTK are Graphical User Interface libraries used by software developers to add standard user interfaces to applications on Linux, Windows, and OSX. The reasons for choosing one are loosely based on fashion, practicality, and the details of the various releases of each library. There are also alternatives popping up faster than you can evaluate. There are good arguments for both.