The ADS-B Out situation with gliders in the USA is complex. Many high-performance modern gliders used for racing and cross country flying are equipped with Mode-S transponders with 1090ES Out (either 14 CFR 91.227 ADS-B Out compliant or TABS/TSO-C199 ADS-B Out compliant) as well as the glider specific FLARM traffic awareness system that broadcasts and receives proprietary low-power position data. FLARM systems, at least as used in the USA also typically receive 1090ES In so they drive the integrated traffic display and warning systems in these modern glider cockpits. For historical and technical reasons gliders use a FLARM modified NMEA serial data protocols for traffic data in the cockpit instead of what is found in GA aircraft ADS-B traffic display systems.

Gliders potentially face very different risk scenarios and their owners will hopefully equip for what is the most significant risks to them and others. e.g. gliders often fly close to each other, especially when thermalling together, if flying with other gliders in remote areas then the FLARM system optimized to handle glider on glider threats is optimal (where GA focused ADS-B produces far to many false alerts), if flying near lots of GA aircraft ADS-B Out (and In via FLARM) is likely optimal, if flying near airliners, fast jets and tactical military aircraft then transponders alone even without 1090ES Out may be most critical item for their SSR, TCAS and IFF compatibility. Ideally owners do equip with all three... Mode-S, 1090ES Out, and FLARM.

The challenge may be more where there are low-cost/low-value gliders, maybe trainers and glider club owned gliders especially those located in busy traffic areas. You would hope those owners long ago got the message they should be equipping with some forms of supplementary traffic broadcast/awareness systems.

UAT adoption in gliders is nearly non-existent as FLARM systems only receive directly on 1090ES In and you don't want to rely on ADS-R coverage in remote or mountainous areas.

TABS/TSO-C199 is an easier route to an approved installation of ADS-B Out systems in type-certified gliders. TSO-C199 was developed by industry and the FAA following the 2006 mid-air collision between a glider and Hawker business jet near Minden NV. Experimental category gliders will typically have 14 CFR 91.227 compliant installations done under the same "meets performance requirements" clauses as many experimental power aircraft.