In the current beta-phase, a minor release is planned roughly every month. After version 0.9 and once no major bugs are remaining, we can release version 1.0. This would then replace the gtk version as default. Once this has happened, vimiv should strictly apply semantic versioning.
The Gtk version will retrieve critical bugfixes at least until version 1.0 has been released. No new features will be implemented there, though.