Smaller than that of a normal aircraft, but very far away from a VLO/stealth one.
The RCS of various light aircraft has been measured and published here: https://apps.dtic.mil/dtic/tr/fulltext/u2/a029262.pdf
The RCS of the human body is on the order of -20 to -5 dBsm: https://ieeexplore.ieee.org/document/7027834
The glider can't hide the human inside. Extrapolating light aircraft RCS down to a glider, my best guess for the lowest-RCS gliders would be on the order of -10 to -5 dBsm, and no less than -12 dBsm or 0.05 m² due to the pilot.
The reason is that a glider still contains a lot of non-radar-transparent parts, which will reflect radar frequencies and produce a sizable RCS.
It doesn't take much to produce a radar reflection. -40 dBsm corresponds to a sphere just 1.4 cm in diameter. Anything that contains metal or water will reflect radar. Even small metal or flesh objects will produce a significant return.
To get seriously low RCS, such as -20 to -30 dBsm, below a bare human, where "real stealth" is considered to begin, you need to specifically work for it, intentionally reflecting the waves in the wrong direction, not avoiding reflections.
In particular, you need to ensure the reflection is directed into specific lobes. This isn't best achieved by radar transparency - in fact, both the F-22 and even non-VLO fighters like the F-16 intentionally coat their canopies with gold to make them radar-reflective, so as to avoid random reflections from the objects inside.