I finally got mine installed and working after mega hours of effort and multiple downloads.
One tip I'll give you both for installation, don't follow the manual exactly. Where it says to delete everything inside the "vehicles" folder except for the HistoricGT folder, don't do it. Only delete other folders, not the files, some are still needed. They know it is wrong but haven't updated the manual yet.
The problem with this mod is that to find any support for installation problems at the usual sites, you have to wade through multiple pages of crap about legality to find the odd problem solving. I found others with the same problems as me but damned hard to find answers. The best place is to go to their home site forum. I feel stupid for not doing that first.
Anyway, hopefully your installation goes smoothly.
I know everyone is raving about it, and some things look very good, but my first impression is average. The driving model of GTL is far superior to me, but I'm pretty sure it is to do with the FFB. For some reason I just can't get the G25 to feel right. This mod relies on the ReelFeel plugin but for me it feels all wrong. It has this large deadzone feel in the centre of the wheel movement which makes all rFactor mods feel like laggy mush to me. GTL is so precise and realistic.
One day, I'll get the FFB better and then I'll enjoy it.