thanks! nix-tree is really great! didnt know about it - thank you. …so it looks like xdg-desktop-portal-gtk is a part of system-path with dbus-1 and user-units as immediate parents. no idea what is that supposed to mean…
im sorry, i dont really understand what do you mean by “use nixos-option environment.systemPackages to figure out what module adds it in”…? but i dont have it IN my systemPackages list…? i am completely lost…
P.S. is this some sort of a fallback portal with pre-installed gnome software? could be useful but not for me…
P.P.S. for context, i only want to leave my hyprland and lxqt environments, and i had this problem with some file openers where some gnome apps kept opening instead, with different interfaces and all. afaik they can be managed either via XDG portals or via XDG MIME associations, which i have both set. but that really freaked me out because i never installed any gnome stuff - is it not possible to NOT have the gtk portal installed and running? why do people then bother installing it as an extra portal, if it comes preinstalled anyway? i would have never thought i would ask such obscure questions… anyway i was going to install a third portal, xdg-desktop-portal-termfilechooser (update: i miserably failed, but will try this), and i really want to keep it minimal, because i heard that its better to have less environments and portals, generally… but im getting distracted! what do i do next? 
EDIT: okay. i have read a bit about portals. the xdg-desktop-portal-gtk is a generic backend implementation FOR the xdg-desktop-portal portal which can be used as a fallback. sure, but i dont need this “fallback”, especially if its overriding my preferences! and i dont want to tinker with the xdg.portal.config preferences either! i just want to simply delete it…