The following car packs are in use by GBGTL leagues and events run by Pete Stenning on his GBGTL server:

It is recommended that you install ALL packs linked below, even if the league you are in doesn't need them.
If the cars are on the server someone can choose them (by accident) and you'll get a crash to desktop if you haven't got that one installed!

The packs were released in the following order (roughly), and some files are updated from pack to pack (paint job re-working for example).
Install in the order of release, overwriting existing files if necessary, and all should be well. No original game car files are altered in any way.

Extract the .zip or .rar archives to your GTL folder using folder names.
I suggest using  7-zip  if you have nothing else with which to extraxt the files. (It's free...which is always a bonus.)

p.s. Once installed, you only need to 'buy' the cars you wish to drive, not any cars driven by others.

1 - Mini7 League Season 1 Car Pack

2 - 'Le Mans 12Hr Event' Enduro Car Pack

3 - Miscellaneous 'Fun' Cars on the server - not necessarily used in any leagues

3a - Miscellaneous Lotus Elites on the server

4 - GBGTL 'Enduro League' Car Pack

5 - GTC-76 League Season1 version2 (Including Sid's late entry BMW No:XXI)

6 - Mini 7 League Season 2 carpack (you will also need carpack number 1)

7 - Elites vs Abarth vs Jag MkII League carpack 1

8 - Mini 7 Season 3 carpack (You also need packs 1 and 6)

9 - Mini 7 Season 3 update (You also need packs 1, 6 and 8)

10 - Mini 7 Season 4 update - just one car to add, number 999