Two goals from Adebayo Akinfenwa and one apiece for Jason Crowe and substitute Giles Coke provided the highlights for the home fans, but followers of the Gills will be smarting at the fact that it was Jackman, who left their club in the summer, that pulled so many of the strings in midfield.
Shell-shocked boss Mark Stimson only took over in November and admitted he was flabbergasted that Gillingham had released such a talented player as Jackman, saying: "He is my sort of player as, although he is small, he performs with a lot of enthusiasm and is always busy."
Northampton were bouncing back from Tuesday's injury-time defeat at Yeovil and still entertained faint play-off hopes, although goal hero Akinfenwa, who has now bagged four in a fortnight, went off with a strained ligament after an hour.
Crowe put the home side in front with a terrific right-foot pass in the 44th minute and it was 2-0 inside a minute when Akinfenwa headed in a cross from Andy Holt.
It was one-way traffic after the re-start with Akinfenwa making it 3-0 in the 53rd minute when he collected a pass from Poul Hubertz and took his time before placing a shot past Simon Royce which hit the post and rolled along the goalline before going in.
Gillingham knew their luck was firmly out by then and they had further confirmation after Coke replaced Akenfenwa and the midfielder saw his shot from the edge of the box deflected over a hapless Royce and dipping under the crossbar.
Gillingham's efforts were few and far between and they were ironic cheers from the visiting supporters when Adam Miller's long-range 70th minute strike at least kept Mark Bunn awake in the home goal.