Ok,i tried to make it in rfactor and it worked for me...
I used tarmac texture,duplicated it(same whit gravel texture!) and selected roada and road as material and surface...Didnt do anythig else!
Then i changed this in tdf!(You can try to copy paste if it is the same in GTL?)
[TRACKVARS]
RoadDryGrip=0.75
RoadWetGrip=0.60
RoadmetalGrip=0.80
RoadDustGrip=0.90
RoadBumpAmp=0.010
RoadBumpLen=13.0
RumbleDryGrip=0.95
RumbleWetGrip=0.60
RumbleBumpAmp=0.015
RumbleBumpLen=10.0
MiscBumpAmp=0.08
MiscBumpLen=4.0
////////////////////////////////////////////////////ROADS////////////////////////////////////////////////////////
// Smooth oval track (this feedback is first so terrains pick up this material match first)
[FEEDBACK]
Dry=RoadDryGrip Wet=RoadWetGrip Resistance=5000.0 BumpAmp=0.022 BumpWavelen=RoadBumpLen Legal=false Spring=0.0 Damper=0.0 CollFrict=0.8 Sparks=0 Scraping=0 Sink=0.017 Sound=gravel
Reaction=dust Tex=Dustroad.tga Max=128 Scale=(1.0,1.0,1.0) Growth=(4.0,3.0,2.0) ASDEnvelope=(2.0,0.2,3.5) DestBlend=InvSrcAlpha SrcBlend=SrcAlpha
Reaction=softskid Tex=skidbrown.tga Max=1024 Pixel=NoReduceDetail Particle=Plane+Deformable+SingleSided DestBlend=InvSrcAlpha SrcBlend=SrcAlpha
//Reaction=wetskid Tex=skidwet.tga Max=1024 Duration=0.40 Pixel=NoReduceDetail Particle=Plane+Deformable+SingleSided DestBlend=InvSrcAlpha SrcBlend=SrcAlpha
//Reaction=dust Tex=dustroad.tga Max=128 Scale=(0.5,0.5,0.5) Growth=(0.8,0.8,0.8) GrowthVel=(0.08,0.09,0.30) Power=0.41 RampSpeed=80.0 OffsetVel=0.10 ASDEnvelope=(2.0,0.2,3.5) DestBlend=InvSrcAlpha SrcBlend=SrcAlpha
Materials=roada,roadb,rdaxroa
// Roads
[FEEDBACK]
Dry=RoadDryGrip Wet=RoadWetGrip Resistance=5000.0 BumpAmp=0.022 BumpWavelen=RoadBumpLen Legal=false Spring=0.0 Damper=0.0 CollFrict=0.8 Sparks=0 Scraping=0 Sink=0.017 Sound=gravel
Reaction=dust Tex=Dustroad.tga Max=128 Scale=(1.0,1.0,1.0) Growth=(4.0,3.0,2.0) ASDEnvelope=(2.0,0.2,3.5) DestBlend=InvSrcAlpha SrcBlend=SrcAlpha
Reaction=softskid Tex=skidbrown.tga Max=1024 Pixel=NoReduceDetail Particle=Plane+Deformable+SingleSided DestBlend=InvSrcAlpha SrcBlend=SrcAlpha
//Reaction=wetskid Tex=skidwet.tga Max=1024 Duration=0.40 Pixel=NoReduceDetail Particle=Plane+Deformable+SingleSided DestBlend=InvSrcAlpha SrcBlend=SrcAlpha
//Reaction=dust Tex=dustroad.tga Max=128 Scale=(0.5,0.5,0.5) Growth=(0.8,0.8,0.8) GrowthVel=(0.08,0.09,0.30) Power=0.41 RampSpeed=80.0 OffsetVel=0.10 ASDEnvelope=(2.0,0.2,3.5) DestBlend=InvSrcAlpha SrcBlend=SrcAlpha
Materials=road
It was funny to see so much dust on tarmac lol,and brown marksXD
You will see that i have added "Sink=0.017" in second row!That ?parametar?(my english sucks lol) was not under road!But it works fine
So if it works you can play whit numbers a bit more.No need to close GTL,just exit race and enter again when you finish editing!(If it works as rF:S)
Hope it helped!