Gretna must have been wishing that the snow which had delayed their journey to Inverness in the build-up had led to the game being postponed as they went in at the interval 3-0 down.
After a fairly even first half-hour, Caley Thistle took the lead on 34 minutes when Russell Duncan surged forward from midfield to set up Marius Niculae and he wasted no time in dispatching a 25-yard effort into the corner of Tony Caig's goal.
Six minutes later and Inverness were two in front. Ross Tokely's cross looked perfect for Niculae to get on the end of but he was hauled down inside the box by Chris Innes.
There was little complaint from Gretna as referee Dougie MacDonald pointed to the spot and John Rankin made no mistake with his penalty, sending Caig the wrong way.
The SPL's bottom club already had a mountain to climb when they fell three behind after 43 minutes.
Don Cowie's corner found an unmarked Tokely inside the box and he had all the time in the world to set up Niculae, who hit a first-time shot beyond the stranded Caig for his second of the match.
Surprisingly, Gretna manager Davie Irons declined to make any changes at half-time and they almost reduced the deficit, but Gavin Skelton's goalbound effort was blocked by Grant Munro with goalkeeper Michael Fraser out of position.
At the other end, Duncan tried his luck from 25 yards with a wonderful effort that required an equally good save from Caig.
With Caley Thistle comfortably in front, the game was in danger of petering out, although Dennis Wyness was inches away from adding a fourth after getting his head to Richard Hastings' cross.
This helped to stir Inverness into action and Rankin had his curling shot turned away by Caig, before the increasingly busy Gretna goalkeeper superbly denied Markus Paatelainen with a point-blank save.
David Proctor came closest to a second-half goal for Caley Thistle, but his flick from close range bounced back off the crossbar.