If you can do a 500 km flight in a glider you are a skilled XC pilot!
The place you are gliding at can explain what the XC endorsement involves, but it's about making sure you have had the necessary training before getting out of glide range rather than doing actual XC flying. That comes with your Silver badge, for which you have to do a 5-hour flight, do a height gain of at least 1,000m, and do an XC flight of at least 50 km. These could all be done in one flight (some do) or in 2 or 3 flights.
But you don't need Silver for the BGA's GPL.