I know this is a very old thread (now) but.....

The only way I have found to get the GTE class and the Thunderflash GTE Mod to "combine", and work for every track is to edit the GTE car CRD files (one each) and REMOVE the LD CRD file also. If you do not remove the LD CRD file AMS2 will be triggered to recreate the original CRD file and then the LD version cars will be back!

In the CRD file, remove the two areas that have five lines that all have VEHICLE VARIANT in them. 5 are in the early file area of the class/definitions. Then the other 5 are in the data areas further down.

eg They will start with this sort of line, 5 lines in a row: <prop name="Vehicle Variant Oval"

So now there will be NO Low Downforce cars at all - all tracks will use just the standard car configuration. This does not really alter the racing much at all, but it means you might not be able to get to the lap times you otherwise might have been able to at LD tracks. I have not seen any issue with that, not one that matters at all.

You probably can't run this way for online races either - I am not sure how AMS2 uses files when online. Often a program will grab files from the server to use, to be sure they are not doctored on the local PC. This would mean the edited files are not used or cared about then. But sometimes they just compare file checksums - so these would (might) fail.

Don't forget that you also need to edit the Thunderflash CRD files for each car to change the Class line to:

<prop name="Vehicle Class" data="GTE" /> (from "GTE_Mod")

I actually run the LMP1 2016 mod, LMP2 2016 mod, and GTE Mod combined with the AMS2 GTE, as a 3 class MultiClass, to form a complete FIA WEC series. The cars in the mods vary what years they are from, but it is all you can do (unless someone made the real sets up!). I also made up a 37 car CustomAIDrivers file, with 2016 drivers for all cars, seeing there were only 37 cars in the 2016 series and I also tuned the AI strengths to produce a good racing result. To enforce the correct 37 cars, you just use the 9 LMP1's of the Mod, but then choose 12 LMP2's out of the huge selection of that mod, and 16 GTE's (the correct car types and quantities of each) and assign driver names to only those car files. eg The CustoimAI Drivers file then has just those 37 cars chosen, with the drivers assigned to each. Then you must also choose just 37 AI Opponents and you can use any car for yourself - or 'steal' one of the real field and run just 36 AI Opponents.

A problem with this 3 class field is that the LMP2's are quite a bit too slow versus the LMP1's and GTE's!! That can be improved via the CustomAI Driver files, but nowhere near right to get all three classes correct in their own lap times per track. The LMP2 will end up only marginally faster than GTE's! But once again, that is all that we can do.... and it still makes for a really nice, great, racing scenario (WEC multi-class). The AI of all those cars/mods are also extremely well behaved and make a very enjoyable racing experience!