When I first got the GRiSO, it had Pirelli Scorpion tyres fitted, these are , I guess, a sort of dual purpose tyre. I also live in the country with a lot of dirt road stretches. I found the scorpions really good in the gravel as they have a wide tread pattern, also ok on the black top, however did not last very long. Since then I have gone for the Michelin Pilot Road, 3's and 4's, running 4's atm and still on the same differing road surfaces, no problems as long as you take care.
The GRiSO's a heavy lump of machinery, and the wide bars make all the difference on skatey surfaces, so far the PR4's have kept me upright.