The Watford loanee, who has scored 12 goals for the Shrimpers so far this term, was forced to sit out Saturday's defeat at Burton with a shin and ankle injury.
Assombalonga will undergo a fitness test ahead of the Gills clash, and is expected to go straight into Paul Sturrock's starting line-up if passed fit.
Barry Corr replaced Assombalonga in attack, but after the Blues saw their 14-match unbeaten run come to an end at the Pirelli Stadium, Sturrock will be hoping to revert to his winning formula.
Mark Phillips made his first appearance following a knee injury as a second-half substitute against the Brewers and is pushing for a start in defence.
David Martin and Freddy Eastwood also featured from the bench, giving Sturrock further alternatives.