A .gtp package is a gzip-compressed-tar archive containing an index.theme file and some theme components. The description of the MIME type has been implemented in the gnome-control-center package originally, following the shared mime info specs, and it's just an inheritance of a .tar.gz MIME type with its suffix changed to .gtp (see definition here). On GNOME the default behaviour when double click is launch the theme manager in theme installation mode.

This are the contents of the .gtp package.

/{theme_name}/index.theme
/{theme_name}/gtk-2.0/
/{theme_name}/gtk-2.0/gtkrc
/{theme_name}/metacity/
/{theme_name}/metacity/...

An example index.theme file to display it's format

[Desktop Entry]
Name=example theme
Type=X-GNOME-Metatheme
Comment=
Encoding=UTF-8

[X-GNOME-Metatheme]
GtkTheme=Gilouche
MetacityTheme=Gilouche
IconTheme=gnome

BackgroundImage=.local/share/wallpapers/Naptime.jpg
MonospaceFont=
ApplicationFont=
DesktopFont=
SoundTheme=
GtkColorScheme=