Comment 2 for bug 2055226

Both options and a third exist.

Extract from man mount in section FILESYSTEM-INDEPENDENT MOUNT OPTIONS:

       user
           Allow an ordinary user to mount the filesystem. The name of the
           mounting user is written to the mtab file (or to the private
           libmount file in /run/mount on systems without a regular mtab) so
           that this same user can unmount the filesystem again. This option
           implies the options noexec, nosuid, and nodev (unless overridden by
           subsequent options, as in the option line user,exec,dev,suid).

       nouser
           Forbid an ordinary user to mount the filesystem. This is the
           default; it does not imply any other options.

       users
           Allow any user to mount and to unmount the filesystem, even when
           some other ordinary user mounted it. This option implies the
           options noexec, nosuid, and nodev (unless overridden by subsequent
           options, as in the option line users,exec,dev,suid).

gio mount is the command line variant to simulate a mount by a GUI tool like Nautilus, just used to get the error message.
gio invokes udiskctl under the hood.