i went through this last year with my glider. the operating
limitations at that time limited me to 91.205 for day VFR operations
and prohibited night/IFR ops. Even though 91.205 on its own does not
apply to gliders, since i was bound to it through my operating
limitations it did apply to me.

Last spring I got my operating limitations changed to reflect the
standard limitation of no instrument requirements for day VFR and
91.205 for everything else.