I have the basic REI Coop handlebar bag on my gravel bike - it is fine. The zipper is probably not water proof, but I put anything that needs protection in a ziplock anyway. It holds a couple of tubes, co2 cannisters, levers and a multi-tool quite easily. For normal rides on smoother / less technical gravel, I carry my phone and gels in it with one tube / co2. For longer, tougher rides, I add a frame bag.
I know several people with nicer handlebar bags - I can see the appeal. But... a few of the ones I looked at were unavailable whereas the REI one was in store, and given the cost, if it wears out quickly I won't be overly depressed.