I have found out that there are not really any good (or any at all) Gavril Grand Marshal modifications except the widebody edition which isn't even being worked on anymore so I hope there might be more eventually. (And it is my favorite car)
I have already found 3 police versions. I mean like tuning / racing version or something like that. But thanks anyways.
Gavril-Grand-Marshal-Widebody-Edition(V0-2) It's quite old now, Not too sure if it works in experimental but give it a try.
I'm assuming you didn't read the op since he mentioned that specific mod in the post. This was Tapatalk's idea, not mine.
What's the point? Apart from the paintjob it would be otherwise exactly the same as the base model. And so would be a pizza delivery, security or a substitute vehicle you get from your dealer when your car is being repaired. You want taxi then paint it yellow and use your imagination, after all that's what sandbox games are all about.
There is a taxi mod... http://www.beamng.com/threads/6634-Chris-his-Emergency-vhicle-shop-CLOSED/page6?p=90858#post90858