The only way to get a guaranteed Gravel is to breed from a Super Gravel. Otherwise I have yet to see anyone that can tell the difference between a Yellowbelly and Gravel.
Better to buy a male Highway (Gravel + Yellowbelly), I see a male for sale for $600. Highways are 'allelic' meaning that all the offspring would be either Gravel or Yellowbelly (acts sort of like a super). I also see a Lesser Pastel Pinstripe Yellowbelly female for $200, so you could get into it for around $800. With that mix you would actually get 32 different possibilities. The hard part is that you couldn't tell if the offspring had Gravel or Yellowbelly for sure, that's what makes it a tough project to get into. Personally I like combos that have clear visual results with no guessing. I think the prices remain fairly high on these combos because the guessing turns off a lot of people and limits the supply. The only way I'd get into this project is if I bought a Super Gravel and an Ivory to get a whole clutch full of Highways, but at current prices that would set me back about $5,000.