There is no need to have gdm3 package installed within PureOS, as based on my guess (as I understood your question) and therefore: sudo apt purge gnome-tweaks. Anyway, please check this post: Byzantium configuration - #11 by Quarnero (when/if applies/needed there).
Otherwise phosh_0.29.0-1pureos1~byz1 doesn’t change anything (in terms of your question).