This friendly fixture represents the first of pre-season for the Gills ahead of the 2024-25 campaign, while the Lions were in action against Bromley earlier this week.
Match preview
Gillingham are preparing for their third successive season in League Two after finishing 12th in the fourth-tier standings last season, six points behind the playoffs.
Just two days after drawing 2-2 with Doncaster Rovers on the final day of last season, the Gills sacked head coach Stephen Clemence after almost six months in charge and replaced him with 38-year-old Mark Bonner at the beginning of May.
Bonner knows what it takes to secure promotion from League Two having previously guided Cambridge United into the third tier in 2021, and he will be tasked with building a squad capable of securing at least a playoff finish in 2024-25.
Gillingham's players reported for pre-season testing and training at the beginning of this month and are now gearing up for their first friendly fixture against Millwall, whom they have beaten just twice in their last 11 competitive meetings.
After taking on the Lions, the Gills will face Watford, Dartford, Southend United and Woking in warm-up matches before hosting Carlisle United in their opening League Two fixture of the new season on August 10.
Millwall, meanwhile, are preparing for their eighth consecutive campaign in the Championship after finishing 13th in the table last season, 11 points behind the playoffs and nine points above the relegation zone.
Almost five years on from his first spell in charge of the Lions, club legend Neil Harris was re-appointed as head coach in February, following Joe Edwards's unsuccessful three-month stint at the helm, and he won eight of his 13 games in charge at the backend of the campaign to steer the London club away from the relegation picture.
Harris's squad have since linked up for pre-season and played out a 0-0 draw with non-league Bromley in a behind-closed-doors friendly at Calmont Road on Tuesday - an "excellent exercise" for the Millwall boss as a number of his players 'got minutes in the legs'.
The Lions, who will be targeting a playoff spot in the Championship next season, will follow up their friendly meeting with Gillingham this weekend with encounters against Charlton Athletic and Reading, before heading off to Spain for a week-long warm weather training camp with two fixtures scheduled at the Pinatar Arena against Nottingham Forest and Real Murcia.
Millwall will then return to England for games against Colchester United and Southampton, before kick-starting their 2024-25 Championship campaign at home against Watford on August 10.
Millwall friendly form:
Team News
With this being the first pre-season friendly for Gillingham, it is unclear how strong Bonner will go for Saturday's contest.
The Gills have confirmed that Scott Malone has joined Crawley Town, while the likes of Macauley Bonne, Shaun Williams and Josh Chambers have all left the club.
Elliott Nevitt is one of four signings made by Bonner's side so far this summer, along with Armani Little, Aaron Rowe and Jack Nolan, and the quartet could feature in some capacity this weekend.
As for Millwall, Harris fielded two separate lineups in each half against Bromley, with all squad members available coming through 45 minutes each.
Harris has confirmed that defender Wes Harding took part in that friendly after returning from international duty with Jamaica at the 2024 Copa America.
Millwall have completed the permanent signings of defender Japhet Tanganga - who spent the second half of last season on loan with the Lions - and striker Macaulay Langstaff, and both players could take part against Gillingham.
We say: Gillingham 1-2 Millwall
It is difficult to predict which team will come out on top in this contest as much will depend on the quality on display from both teams, but Millwall will be regarded as favourites if they are to name a relatively strong side on Saturday.