I contacted the Dutch KEI and they essentially told me that the GPL that they gave me (in exchange for some money) holds no legal value since the Netherlands, just like the UK, doesn't require any form of licence to fly gliders. It's just a bit of paper to make you - and possibly foreign gliding centers/authorities feel good.
I feel a bit cheated now... But anyway, yes, at this time this means that I don't need to convert anything to an EASA SPL(S) or LAPL(S). Pfew.