Neil Harris’ side were relegated to League Two on goal difference after losing to Rotherham on the final day of the season.
And as part of their pre-work for life in the bottom tier, the Gills have agreed to host the Blues at the Priestfield Stadium on Saturday, July 16 (3pm kick-off).
The friendly will be played less than three months after the Blues helped relegate Harris’ side, following a 3-1 win at Fratton Park on April 23 – Pompey’s penultimate home game of the 2021-22 campaign.
To beef up his troops preparations for a sixth successive season in League One, Danny Cowley will also take his squad to a trio of local non-league outfits.
After two weeks of work on the training pitch – with the players reporting back for pre-season duty on Monday, June 20 – the Blues’ first game will be their traditional trip to the Hawks at Westleigh Park on Saturday, July 2 (3pm).
Pompey will then go to Privett Park the following day (Sunday, July 3) to take on Gosport Borough at 3pm, with the squad likely to be split across the two fixtures.
After that, the Blues will fly out to Spain and be based in Murcia for a warm-weather training camp.
That trip will be concluded with a game, with more information to be announced in due course.
Meanwhile, a Pompey XI will travel to Nyewood Lane to face Bognor on Wednesday, July 20 (7.45pm).
The Blues are also finalising plans for a fixture at Fratton Park on Saturday, July 23, along with other friendly fixtures.
That PO4 game will be held a week before the new League One campaign begins on July 30.
It will help serve as a test event for the work that is currently ongoing on both the North and South stands.