So, you'd log that flight in the airframe log, but not in the engine log?
And in your personal logbook you log it in the "Glider" column, and the hours cannot be used towards SEP revalidation...
Logbooks specific for gliders have a launch method column though. Winch, Tow and one other which I forgot. I wonder how you put "gravity assist" AKA freefall in those.