Mike Hallum and GeeWhiz, couple of the points you raise need further thought on the part of the intrepid cross country power pilot.
First, calling the gliding ground frequency will probably get no answer at all. They don't work on radio, chaps. If a glider has to land back at the airfield, he has to land, so no point asking permission. Result, if the lift peters out, everybody comes back at once, and they avoid each other by lookout and good judgement, not by talking on the radio. Few gliding clubs have somebody sitting at the launchpoint to listen out and very very few gliding clubs are entitled to control traffic. So just because nobody answers your call doesn't mean they are not active.
As for flying overhead to clear potential winch launch cables, o my. Sure, our Skylaunch can lob us up to l,800 feet on a good day, so what do we do next? if we are lucky enough to connect with a thermal, we keep on going up. And up and up, to cloudbase. Directly over the airfield. So depending on conditions, a gliding site like Lasham for instance, will have gliders swarming overhead from the top of the launch to the controlled airspace!
Still think its safe to proceed after a radio call? and still think its safe to fly directly overhead?
Yes, exercise your legal rights, by all means. But if you have no gliding experience or understanding of their peculiar habits, please keep a very good lookout in all uncontrolled airspace, and use judgement when planning your flight. Probably the best plan is to fly above any fair weather cumulus. Less bumpy, fewer gliders.