It would be nice if vim-gtk-syntax is listed as vim plugin in Gnome Software
vim cannot be installed in GNOME Software (presumably as it does not install a desktop file), so I think that there is little point in making vim-gtk-syntax an addon.
Sorry, but I was speaking about gvim of course, which is GTK+ version of VIM and ships .dekstop file of course.
For the reference, recently I did something similar for vim-command-t I maintain.
I added an AppStream metainfo file in Rawhide and F21.