Poul Hubertz stepped off the bench to grab the winner for the second time in four days as Millwall booked their place in the Carling Cup Second Round.
Big Dane Poul, who slotted home the decisive goal against Oldham on Saturday, showed that he is just as useful with his head as he powered home a textbook header from Filipe Morais' corner with just five minutes remaining.
Kevin Braniff had earlier fired Millwall into a first-half lead, capitalising on a 38th-minute slip by Gills' defender Brent Sancho before despatching an inch-perfect effort that curled past the outstretched hands of keeper Darren Randolph and into the bottom corner.
It was Braniff's second goal in three games, and a finish of sublime quality that crowned a really impressive showing by the irrepressible Irishman.
This was a gutsy, hard-working display by a young Lions outfit including debutants Zoumana Bakayogo and Samy Mawene, plus 17-year-old striker Ali Fuseini on the bench.
And when Neal Ardley was forced out of the action at half-time with a rib problem, Maurice Ross was the 'veteran' of the side at the age of 25 until Hubertz's introduction midway through the second half.
By the time Great Dane Poul entered the fray, Millwall were a goal up, but had relied a couple of times on the magnificence of stand-in skipper Paul Robinson to block the Gillingham danger.
Keeper Lenny Pidgeley came to the rescue on 69 minutes when he somehow got his fingers to tip over a Clint Easton piledriver, whilst Morais dragged his shot wide three minutes later from 20-yards.
Gillingham grabbed an equaliser on 80 minutes when sub Andrew Crofts sent a bullet header past Pidgeley.
But instead of allowing their heads to drop, Spackman's youngsters dug deep with Chris Hackett's stunning solo run fully deserving the deafening applause of The Den faithful.
The volume was turned up to ear-splitting level though, five minutes from time, when Hubertz looked to have headed Millwall into the Second Round.
However there was nearly a last-gasp twist to the tale, when Pidgeley superbly got a hand to steer the ball away to safety from Gary Mulligan, who had the goal at his mercy.
But with that save went Gillingham's last chance of the game - as Millwall made it 11 home Cup ties against their Kent opponents without defeat.

Millwall: Pidgeley, Ross, Bakayogo (Craig, 77mins), Elliott, Robinson, Whitbread, Brighton, Ardley (Morais, 46mins), Braniff, Mawene (Hubertz, 65mins), Hackett. Subs not used: Day, Fuseini.
Bookings: Robinson (81mins), Braniff (88mins)
Scorers: Braniff (38mins), Hubertz (84mins)

Gillingham: Randolph, Jackman, Flynn, Sancho, Cox, Jupp, Bentley (Crofts, 57mins), Jarvis, Easton, McDonald (Mulligan, 72mins), Ndumbu-Nsungu. Subs not used: Johnson, Pouton, Clohessy.
Scorer: Crofts (80mins)

Ref: Keith Stroud
Att: 5,040 (Away 576)