OK this feels wierd!! I am going to try and offer computer advice
Please stand well back and use protective clothing!!!
I presume that you have done a fresh install of GPL from the CD using the GPLPS installer? So that you have a virgin GPL with the original 67 carset.
So, does the basic 67 game run? as a standalone game?
I have Gem 2.5.32, which I believe is the latest version.
Will Gem work with the virgin GPL install?
If things only go pear shaped when you try to install the 55 mod......
I trust that you have ensured that you have all the track files listed in the readme section of the 55 download instructions
BEFORE you attempt to install the 55 mod? cos it won't work if you don't. Check that you have the correct version of the Buenos Airies track and the Spa67 track not the original Sierra version?
I had a similar issue with the "error" message recently and it was caused by the xxSEASON.INI files being corrupted. I am suspecting that your 55SEASON.ini file does not have the correct tracks and therefore the 55 mod will not load ?
Of course, this might all be total nonsense.........................and if it is,
I would suggest posting for help on the UKGPL forum as there are one or two members who were involved in the development of this new mod and they should be able to assist Clive.