An air gap for a dishwasher is for the purpose of preventing sewage from a sink or garbage disposal to flow back into the dishwasher and contaminating the dishes. A health concern. Not all states require an air gap to be installed, however, some do. Dishwasher manufactures may have a back flow prevention device installed in their dishwasher, others may recommend one be installed at the sink even if they have a back flow device on their dishwasher.
Normally easy and inexpensive to install
Water running from the air-gap is not unusual and can normally be corrected easily. It usually results from a blockage in the short hose that goes from the air gap to the disposal or sinks tailpiece, being partially obstructed or clogged. Clearing any blockage in the short hose or replacing it normally solves the issue.
Is the disposal brand new ?
If the garbage disposal is brand new and has just been installed; then there is a small chance that the installer forgot to remove a small metal knockout plug where the hose from the airgap is connected. If the installer forgets to do this, the water can not enter into the disposal and thus flows out of the air gap.
If this is the case; just remove the hose connected to the disposal, remove the metal plug and reconnect the hose. Remember to disconnect the electrical plug when you do this.
Who to consult? Plumbers and handymen.
Home owners will often correct water running from the air-gap themselves. Plumbers and qualified handymen will likewise do these plumbing tasks easily and quickly.