GtkIMContextSimple is a simple input method context supporting table-based input methods.
It has a built-in table of compose sequences that is derived from the X11 Compose files.
GtkIMContextSimple reads additional compose sequences from the first of the following files that is found: ~/.config/gtk-3.0/Compose, ~/.XCompose, /usr/share/X11/locale/$locale/Compose (for locales that have a nontrivial Compose file). The syntax of these files is described in the Compose(5) manual page.
GtkIMContextSimple also supports numeric entry of Unicode characters by typing Ctrl-Shift-u, followed by a hexadecimal Unicode codepoint. For example, Ctrl-Shift-u 1 2 3 Enter yields U+0123 LATIN SMALL LETTER G WITH CEDILLA, i.e. ģ.