To access one of the PRACE supercomputers, I was required to use gsissh. The corresponding gsi-openssh-clients package is not in the standard Ubuntu repositories. I have downloaded it from http://toolkit.globus.org/ftppub/gt5/5.2/stable/packages/deb/ubuntu/12.04/pool/contrib/g/gsi-openssh/.Besides that also globus-proxy-utils is needed. This one was easy to install via apt-get install globus-proxy-utils. Finally, everything got working when the bundle of X509 trusted certificates was downloaded from http://software.ligo.org/gridtools/debian/pool/main/o/osg-ca-certs/. Note, for some reason .globus folder did not appear in my home directory. To use gsissh, I had to create it and then move my user certificate to it with proper correct permissions -rw-------.

P.S. Do not forget to create a proxy certificate (grid-proxy-init) before login to your supercomputer.