You are looking at it too complicated. The channels are for the shader.
You UV map your mask to your track
You make your "mask" texture just like any other, just paint red, blue, green where you would like different surfaces to appear.
And it's in the shader (in sdk Editor) that you assign for example old grass detail to the red channel, green grass to blue, dirt to green, whatever.
If you can't solve it till next weekend, I'll try to post a pseudo-tutorial
Sent from my phone using Tapatalk
Yeah sure that would be cool, you will see on the map where the offtrack areas need to beyes all this is just graphical magical wizardry,
I think you should learn the shader first without any interference thinking about modelling. but by using what you have. forget about side transition textures (your on the right track there btw).
this would involve just working on the main scenery with your main google map diffuse. so if you like, if you send me your google map file, ill send you a bunch of stuff back with a tutorial about how to set it up. hows about that ? and through having a working copy infront of you, you can then understand it.
simple mask for that diffuse. features red and green, where red is main grass. where green is, is where your other grass texture should be. just 2 channels used.
https://www.mediafire.com/file/hfzxqmrm1d4mtt8/GoogleMap_mask.rar