I took a quick and dirty way to get fast results.
I copied the xml files into the Mods\GEM+\gpl65 directory.
Next a subdir in Sound\GPLES named gpl65.
There is to create for every chassis a subdir (for gpl65 the same as the 67).
Make sure in the Brabham dir must be a file named gples.ini otherwise the editor does not start for this mod!
Then i copied the BRM sound file for the 65 BRM261 named b61.wav into the GPLES/gpl65 directory.
With Audacity i opened this sound and took the telephone filter. Save as b61off.wav.
Additional i took all backfire sound files i found in NR2003 an copied them all into a single file using Audacity. There i took the filter to set he pitch (i hope this is right, i'm using the german version = Tempo/Tonhöhe ändern) for 50% higher Speed and 50% higher pitch. Saved/exported as backfire.wav.
Have a look into my GPLES.INI attached as a bitmap for the detailed values.
GPL sounds much better with this easy way to make a sound set.