You can connect from outside GSI to GSI linux machines via ssh.
GSI provides a login cluster for ssh : lxlogin.gsi.de
This cluster is not meant to be used as working machines since no software is installed here. You just use the cluster to login via ssh and connect to your target host.
ssh username@lxlogin.gsi.de and afterwards ssh username@mytargethost.gsi.de
or
ssh -J username@lxlogin.gsi.de username@targethost.gsi.de