Jackman has spent most of his career at left-back but was converted into a midfielder during his spell with Northampton and is keen on occupying that role for the Gills.
Jackman said: "As soon as I found out the club were interested, there was only one place I wanted to come back to.
"You hear little rumours of things but you are never too sure whether they are true or not and I had to concentrate on what I was doing on the pitch.
"If the manager needs me to play at left-back I can do that but hopefully I can play in midfield and be more attack-minded.
"I owe a lot to Stuart Gray as he gave me that chance to play in midfield and that is where I put in a lot of good performances."