Just 431 fans watched Gretna keeper Greg Fleming rescue his side numerous times before Roy McBain (71) fired Caley Thistle ahead.
Don Cowie soon made it 2-0 but Craig Barr then gave the fans of closure-threatened Gretna something to cheer by narrowing the Highland side’s lead.
Gretna’s John-Paul Kissock struck the bar from 20 yards on 50.
But Inverness finally added the finishing touches to their play when the ball broke well for McBain 15 yards out and he struck a shot into the corner.
Soon Cowie ran on to a ball over the top and volleyed into an empty net after Fleming had got a hand to his attempted lob.
Gretna pulled one back with 10 minutes left when Barr finished inside the near post.