In a first half dominated by Thistle, it was hard to believe they only went in at the interval just the one goal to the good.
Gretna's youngsters were simply no match for an experienced Inverness line-up and the only surprise was the fact it took the hosts just over 21 minutes to open the scoring.
Player-manager Craig Brewster was the first to go close with a header from Barry Wilson's cross, before Don Cowie's goalbound shot had to be deflected behind for a corner.
No sooner were Gretna clearing the ball than it was coming back at them and they were given another let off when Alan Morgan's surging run down the left resulted in a cross that once again Brewster could not direct on target with his head.
The visitors were simply not heeding these warnings and they were made to pay on 21 minutes.
Wilson's corner found the head of Brewster and his flick may have been heading into the back of the net, but Graham Bayne opted just to make sure by getting the final touch to help it over the line.
After such a lacklustre first-half display, Gretna showed signs of improvement at the start of the second half with a Steven Hogg volley curling just past the upright.
Captain Allan Jenkins also saw his low drive saved at the second attempt by Zibi Malkowski as the visitors searched for an equaliser.
However, their hopes were dashed on 67 minutes when substitute Dennis Wyness pulled the ball back for Barry Wilson to steer into the net from eight yards.
Caley Thistle were now in complete control and they rounded off the scoring 10 minutes from time with a well-worked goal.
A cross from John Rankin was inch-perfect for Ross Tokely coming in at the back post and he diverted the ball across the face of goal for Wyness to tap home his third of the season.