Injuries and international commitments means head coach John Mousinho will be forced to reshuffle his pack for the game against the League Two side.
However, he’ll be confident that the ‘strong’ side he puts out will be a match for the Priestfield outfit.
Here’s how we think the Blues will line up for their second game in this season’s competition.
5. Centre-back 2: Ryley Towler
The fans' favourite has had to take a back seat this season following Conor Shaughnessy's surprise emergence. The 21-year-old has featured just four times this season in all competitions. That amounts to just 253 minutes of football. The Blues' forgotten man will, therefore, be keen to remind people of his quality. Photo: Jason Brown
6. Left-back: Jack Sparkes
The former Exeter man has taken his chance well in the absence of Connor Ogilvie. Ogilvie is on the comeback trail and will likely be fit for the League One game against Carlisle on October 21. Tonight's game will be another opportunity for Sparkes to prove a place back on the bench is a foregone conclusion. Photo: Jason Brown
7. Midfield 1: Ben Stevenson
The summer arrival has had to bide his time for opportunities this season. He was given his first league start on Saturday against Port Vale as he stood in for the injured Marlon Pack. Stevenson took to the task like a duck to water and will be given a rare chance to start back-to-back games against the Gills. Photo: Jason Brown
8. Midfield 2: Joe Morrell
With Marlon Pack and Tom Lowery injured, plus Terry Devlin and Alex Robertson both away on international duty, John Mousinho might have to call upon the Wales international to help plug a midfield gap tonight. Morrell isn't involved with the Welsh this international break because of a suspension. That could work to Pompey's benefit. The only alternative would be to see Tino Anjorin drop back and give Morrell a breather. Photo: Jason Brown