In most countries, for contest flying in gliders, the rules prohibit any kind of attitude indicator or turn rate indicator, to ensure that pilots do not illegally enter clouds and gain an unfair advantage. 

However, some appropriately-rated pilots do legally fly appropriately-equipped gliders in clouds.  In many cases this is done in the context of "wave soaring" at very high altitudes-- above 18,000' -- where the pilot must be in contact with Air Traffic Control and operating under Instrument Flight Rules anyway, unless he is operating within a special "wave window". In the United States under current regulations, an attitude indicator would be required for flight under Instrument Flight Rules.  

There are some countries that still allow cloud flying in gliders without special coordination with air traffic control (i.e. without operating under Instrument Flight Rules) in significant portions of the airspace.  England is one such country.  Here's a publication from a gliding club in England re controlling a glider using only a turn rate indicator, with no artificial horizon -- http://www.dartmoorgliding.co.uk/Instrument_Flying_course_notes.pdf .  It takes some specialized training to be able to carry this off safely.  

Back in the 1940's through 1960's, this type of cloud flying used to be much more common throughout the gliding world than it is today.  The primary reasons for including a turn rate indicator on the panel, but no attitude indicator, were cost, weight, panel space, reliability, power requirements, and resistance to "tumbling" if excessive pitch or bank attitudes were attained.  In some cases the turn rate indicator was powered by a venturi that was exposed to the airflow when a special panel in the side of the fuselage was opened-- though this was undoubtedly a poor solution in cases where icing was possible.  

The "Bohli compass" is an exotic compass for sailplanes that was specially designed to function somewhat like an attitude indicator-- http://www.hkavionics.com/Bohli_man/ba_kompi_e4.pdf .  Some other specialized compasses have been developed for gliding that are less exotic than the Bohli, but do minimize compass errors and help allow the compass to be used as a directional gyro to some degree-- see photo https://www.reddit.com/r/Gliding/comments/6kb7li/what_is_the_name_of_this_type_of_compass/ -- also google "Cook compass".  

Never try anything like this without wearing a parachute.  Gliders have broken up in clouds.  

In terms of state-of-the-art instrumentation, things have come a long way from the old days.  Now there are sophisticated computerized variometers (sensitive rate-of-climb indicators) that incorporate accelerometers as well as pressure sensing, and in some of these instruments the accelerometers can be used to drive a AHRS-based attitude-indicator-type display on the screen of the instrument.  See for example https://www.air-avionics.com/?page_id=552 .  (How is the issue of unfair use in a contest addressed?)  As all pilots know, this is not a simple task-- you can't just dangle a weight on a string (or the electronic equivalent thereof) and find out which way is "up" -- a lot of computer processing is involved -- see for example https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3279199/ .