Gliding examinations have to be taken before going cross-country, even if you have all your PPL exams passed (an ATPL at my gliding club had to do them all too).
In the Netherlands, my PPL gets me waivers for four out of six ground exams. The ones I still have to do are Gliders Technical (construction) and Navigation.
An ATPL would get you a waiver for Navigation too.
But I have to say that the BGA seems to have a totally different examination regime compared to the Dutch KNVvL, so YMMV. Even though they both claim to deliver an ICAO-compliant GPL.