Assuming that you would construct the gravel road in similar fashion as if it were a base course for an HMA road, Indian Highway standards say:
plus/minus 15 mm for base course, manually laid (as opposed to using a paving machine. Their "subgrade" tolerance which would likely be more relevant is plus 20 mm, minus 25 mm. horizontal alignment: plus/minus 25 mm.
This is probably a good start; you can adjust to how critical you think that you should be for your road's purpose.
