The Gills suffered a 3-2 defeat to Oxford United at the weekend, despite being 2-0 up on 72 minutes, and their outside chances of making the play-offs were ended as a result.
A game at second placed Peterborough will be no easy task to bounce back, but here's how we think they'll line up at the London Road stadium.
Although manager Steve Evans switched to a 4-4-1-1 formation for the defeat at Oxford, we expect that it'll be changed back to the 4-4-2 lineup that they have utilised throughout the last few months. However, it could easily switch between those two options or even a 4-1-2-1-2 depending on the situation.
One of the safest bets to make it into the team, as he has started every single League One match this season. Whilst he's let in seven in his last three games, there is very little chance back-up keeper Sacha Bastien will step in, and the Irishman has still kept 10 clean sheets in all competitions during 2020/21.
The first choice right-back for the Gills has been an almost constant presence this season, so it's likely it will remain that way for Tuesday. The danger he poses from his long throws especially gives reason for him to stay in the starting XI.
Bristol City loanee Cundy recently came back from injury and slotted back into his usual centre-back position. He also got his first goal for club in that Oxford defeat, and it's doubtful he'll be swapped out to face the Posh.
Tucker has been a solid presence in the Gills' back line, throwing himself into the fray and showing great commitment to last-ditch defending. He's been Cundy's regular partner and that will remain the same on Tuesday.
The most experienced defender in Gillingham's defence, Ogilvie has proved himself to be one of the most reliable players in the squad. He's also not bad going forward either, having contributed four goals and four assists during the campaign.
The livewire winger has caused havoc all season, creating problems for opposition defenders almost anywhere in their half. In total, the 12 goals and eight assists he had managed demonstrate his enormous ability, and the Gills would be amiss to leave him out.
Gillingham fought off several other League One sides to secure the services of Dempsey last summer, and his performances have shown it was worth the effort. A dynamic centre to the Gills' midfield, his eight goals and five assists have seen a decent return this season.
Since returning to the squad from injury earlier this year O'Keefe has produce hardworking display after hardworking display. Not shy of a challenge, the midfielder has racked up plenty of bookings, but his determination is something to be admired.
The 31-year-old Scotsman has served in nearly all areas of the midfield this season, providing a reliable option in every position. Often completing the less flashy work to connect the defence with the attack it's likely he'll start again on Tuesday.
The first name on the team sheet. The man of the moment for Gillingham, Oliver is having the best goalscoring season of his career. Another strike in the defeat to Oxford was his ninth goal in 11 games
Whilst Akinde might have wanted a greater return than the seven goals he's scored this season, the striker has still popped up in crucial moments to secure some big points. Could easily be swapped out with Olly Lee or Dominic Samuel, but Evans might want consistency to remain as he has partnered with Oliver in nearly every game.