I'm not strongly for or against flats or drops for gravel, but most of gravel riding for me is bikepacking, with long hours on the bike. For long hours, the numerous hand and body positions offered by drops considerably outweigh flat bars.
Hybrid of both
As noted above, flat bars are preferred for descending on mixed terrain but much prefer multiple hand positions to alleviate hand fatigue. Solution: wide drop bars like that of the Salsa Woodchipper