First, calling the gliding ground frequency will probably get no answer at all.
I've flown gliders from Hullavington and Halton on winch and aerotow, both of which have had someone with a radio operators certificate and radio equipment either handheld or base station. I've found many gliding sites that have some kind of radio comms, not for issuing landing clearances to gliders(!) but for the purpose of advising overhead transits of gliding activity.
I fly from a small airfield where the radio operator is not always present; in this instance pilots sometimes provide arriving traffic with a runway, pressures, and approxmate surface wind from their own departure (of course all taken in an advisory manner, but still helpful).
Personally I would not recommend flying over or too near to gliding sites and actively avoid them myself, but. If there is a telephone or frequency conversation that can be had to increase safety and provide a safe transit, why not? Also there are some gliding sites that sit between low altitude CAS and danger areas active to flight level nosebleed. Should the powered pilot not be able to fly in or above cloud this could get sticky. I'm sure a little diplomacy and understanding are all that's needed. Its open FIR, share and share alike.
