I am a builder and suggest that it is more than likely condensation if its opposite this air brick.I am assuming you have a cavity wall?
Older houses weren’t meant to contain the levels of heat modern living requires.
If the wall has had cavity wall insulation or improperly fitted wall ties (original or “new”) this may cause bridging between the two skins, otherwise any water that enters or is absorbed by the outer skin falls to the bottom of the cavity and is removed, via air bricks, by evaporation.
The gravel isn’t the best of ideas, but ain’t the worst:it doesn’t raise the effective ground level as water drains straight through…
Get a builder in to offer advice, don’t ask on here!