To install GX_Simulator as part of the SSW tree one should select the GX_Simulator package on the Solarsoft installation form and follow the general installation instructions.
Alternatively, one may chose to upgrade an already existing SSW installation, using the IDL command line
However, if the latest option is chosen, one should make sure that sswidl.bat (on Windows platforms) or cshrc (on Unix or Mac platforms) scripts are properly updated such as to include gx_simulator in the SSW_INSTR path declaration, i.e.
set SSW_INSTR=gx_simulator [...]
where [...] denotes any other SSW packages installed
In addition, the GX_Simulator installation process may require a few additional OS-specific setup steps.