Fixing gap under one side of a steel garage door
Trough out the high side of the concrete where the door lands on it. 

Get an angle grinder with a diamond blade. Mark the concrete on both sides of the closed door. 

Cut 2 straight lines on high side and chisel/ grind  away a trough equal in depth to the 1.375” gap.

I used a laser to determine how much material needed to be removed.

On my neighbors house it only required about 14 inches of  trough to allow the gasket to make full contact.