As of Version: 1:6.7p1-1 of openssh-server, it appears that Debian [and presumably upstream]'s sshd now has diffie-hellman-group1-sha1 disabled. This means that connections from x2goclient will fail.
I was able to work around this by adding:
KexAlgorithms curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1,diffie-hellman-group-exchange-sha1,diffie-hellman-group1-sha1
to /etc/ssh/sshd_config, but obviously at some point support for diffie-hellman-group1-sha1 is going to go away completely, rather than just being disabled by default.