The home side are managed by former Millwall boss Neil Harris, but the visiting Championship side will expect to prevail at the Priestfield Stadium.
Match preview
On-the-pitch matters at Millwall have been placed very much into the background for the time being after chairman John Berylson was tragically killed in a car accident in the United States earlier this month.
Tributes have poured in for the 70-year-old and his death was marked at a behind-closed-doors friendly with Bromley earlier this week, Millwall prevailing by a 6-1 scoreline.
However, Gary Rowett's job is to ensure that his players are fully prepared for the Championship campaign to come, the Lions left with just four more friendlies.
Sutton United, Charlton Athletic and Fortuna Sittard are all to follow in due course, yet Rowett will want to focus on the upcoming fixture given his opposite number Harris's association with the East Londoners.
Rowett has made two major additions thus far, signing left-back Joe Bryan on a free transfer and forward Kevin Nisbet for an alleged £2m from Hibernian.
For the large part, Gillingham were disappointing throughout last season, eventually finishing in 17th position in League Two, but the club are on the up under Harris.
After seeing off the challenge of Dover Athletic, Gillingham recorded a hugely-impressive 1-0 triumph over Serie B side Como, netting the decisive goal in the 88th minute.
That result came in Italy, representing a notable scalp for the fourth-tier side, yet Harris will want to see repeat performances against their upcoming English opponents.
Max Clark, Conor Masterson and Jonny Williams - each of whom have international pedigree - have all been signed on free transfers.
Gillingham friendly form:
Millwall friendly form:
Team News
Millwall opted not to disclose the team that they fielded versus Bromley, but Nisbit, Andreas Voglsammer, Tom Bradshaw, Duncan Watmore, Aidomo Emakhu and Niino Adom-Malaki all got on the scoresheet.
Rowett has spoken of his plan to mix up his players, combining experience with youth, to take a look at different combinations all over the pitch.
Meanwhile, Harris used two separate teams for the fixture with Como, the goal coming from forward Dom Jefferies in the second half.
The sides named appear to have no baring on who is where in the pecking order. Of the new arrivals, Williams played the first half, with Masterson and Clark featuring in defence after the break.
We say: Gillingham 1-2 Millwall
Gillingham have already shown in pre-season that they can upset the odds against bigger clubs, and they will fancy their chances of replicating previous efforts on Saturday. Nevertheless, we feel that Millwall's superior quality will eventually shine through to hand them a hard-fought triumph.