In addition to ASI, Altimeter and Compass
Turn and slip indicator (approved type in the specified equipment list) or attitude indicator (again a list of approved make/models is specified).
However, I would not enter cloud in a modern high performance glider with only a T&S, although I have several hours in lower performance gliders with only T&S as a gyro.
I am comfortable with a current generation EFIS such as Dynon D10A, but the type certificate requires me to use an AH - RCA26AK-3 for example (which doesn't do as good a job, and doesn't have an interbnal standby supply - such is certification).