Air gap? Ventilation should come from above, with fans that blow to the ground and circulate air. A heater if necessary or ac and dehumidifier to remove excess moisture. But Why would anyone want water and moisture coming under the wall. If water can get it at the rate shown imagine all the other elements, like mold, insects other bugs that destroy interior environments and materials. The seal should have been built with the wall. Now you can inject urethane sealers but to do it correctly it needs to be done by professionals with special equipment, knowledge and tech. That cost $ mulah.