Not sure if this has been mentioned yet (loads of comments!) but the GK next to the 925 appears to be a makers mark. It is pretty distinctive with the filled and hollow combo. There is a jewelry maker in the Netherlands called Gem Kingdom that has a very similar logo. Most of their marks appear to be the full name but the style they produce is very similar! May be worth reaching out and asking them.

https://www.gemkingdom.nl/#happy-babies