First locate the file [GPL]\Mods\Gem+\gpl55.cst
where [GPL] is your base GPL directory where gpl.exe is installed.
Rename that file to back it up (in case something goes wrong), making sure that you change the file extension. I suggest a name such as "gpl55.cst.original". Now copy the new version of the file from the attached ZIP file into that directory. Next time you start GEM, it should recognise setup files for all the 1955 mod cars.
P.S. Thanks to Bob Simpson for posting his "All GPL Mods Info" spreadsheet. I was going to check the 1955 mod setup extensions as I tested the cars, but his spreadsheet lists the info already and saved me some time. Cheers Bob.
Edited by Saiph, May 02 2017 - 12:59 PM.