GTK was initially developed for and used by the GIMP, the GNU Image Manipulation Program. It is called the "The GIMP ToolKit" so that the origins of the project are remembered. Today it is more commonly known as GTK for short and is used by a large number of applications including the GNU project's GNOME desktop.
Peter Mattis started it all, with help from Spencer Kimball and Josh Macdonald. Marius Vollmer taught us how to make GTK work for language bindings. Lars Hamann and Stefan Jeske added all sorts of features to the core widgets. Carsten Haitzler designed and did the first implementation of theme support. Shawn Amundson did release management for GTK 1.0 and GTK 1.2.
Alexander Larsson releases Glick, his first app bundling framework
Glick 2 is released, a modernized version of the first Glick framework
“GNOME OS” session held at GUADEC, includes initial planning for a new app bundling format
Alexander Larsson releases experimental “bundler” framework
“Linux Apps” proposal discussed at the GNOME Developer Experience hackfest, Brussels
Work begins on xdg-app, which later becomes Flatpak
xdg-app 0.1 released, the very first version of Flatpak