I had exact same problem installing soapUI on my 64 bit Ubuntu 12.04 (precise) system.
First, I uninstalled OpenJDK and installed Oracle/Sun Java 1.7_07 iaw. recommendations on other sites, but to no avail. Still getting error message 'Could not display the GUI. This application needs access to an X Server'.
What solved the problem was installing 'ia32-libs' (which has a dependency to several packages, among them 'ia32-libs-multiarch:i386', which is also installed). After running the installer shell script again, I still got some error messages (ELF versions etc.), but installer GUI actually started anyway. Running the installed (with sudo bash install_script.sh) again, the error messages from previous install were all gone...
Thanks for tips!