Generally, while running the gpseinstall, the default user ID is gpadmin but if the user wants to add a different group ID rather than the default user ID, there isn't an option to do so in the --help section of gpseginstall.
There is an undocumented -g option that will allow you to add the system user with a different group ID instead of what is specified for the user ID.
The command below will add a different group ID during the process of adding the system user when running the gpseginstall process. The only caveat is that you would have to manually create the group using the groupadd command in Linux before running the gpseginstall command.
gpseginstall -f /hostfile_exkeys -u testuser -g testgroup -p P@$$word