We currently do not allow manipulating gvfs/gio in this manner. The snap should typically instead plugs removable-media or possibly udisks2. Is there a specific reason why automount by udisks2 isn’t working for you?
You can plugs the desktop interface then use xdg-open /path/to/file/or/dir (be sure to use /usr/bin/xdg-open from the runtime and not from inside your snap).
This is not typically a justification for classic. The mountpoints may be in /media or /mnt when using removable-media.
If you haven’t seen it already and for criteria we’ve seen before in this forum, Process for reviewing classic confinement snaps lists supported and unsupported criteria for use of classic.
