Hello Saion,
as a quickfix you can use another gpg server: apt-key adv --recv-keys --keyserver keyserver.ubuntu.com E1F958385BFE2B6E
On debian buster-backports and newer systems you can use extrepo: apt install extrepo && extrepo enable x2go && apt-get update && apt-get install x2goserver
I have changed this in wiki.
Best Regards, Juri Grabowski