Considering the vast improvement in performance of modern gliders, plus things like GPS I would not regard the GPL as proving very much. Even in my time many moons ago, we always said the the only worthwhile qualification for a glider pilot was Silver C. I ran out the statistics at three clubs I was in, and only one in ten of the people who joined the club as ab-initios made it to Bronze, never mind Silver. I doubt more than 5% made it to Silver in those days.
I think that the Silver C tasks need revising in any case. I would drop the five hours altogether which proved nothing except the strength of your bladder. I did it in thermals, because I thought that was the spirit of the task, but most folk snuck off to a ridge site. The distance ought to be at least !00km maybe even 150km, and the height gain at least 2000m. Achieving that standard ought to be possible these days. I'm thinking of having a wooden surround made for my Silver Badge to make the point.