Darren Byfield blasted a second-half hat-trick as Millwall marched to their biggest home League win since the 4-1 hammering of West Ham in March 2004.
And you have to go back to the Second Division title-winning season to record a Lions player scored three goals in a home game, with Paul Moody enjoying that honour in a 3-1 win against Wigan Athletic on 13th January 2001.
It proved to be a thoroughly satisfying afternoon for Willie Donachie's men, who followed up the Boxing Day battering of Brentford with another four-goal show.
Most important though, this win has seen Millwall move to sixth from bottom of the League One table, two points clear of the drop zone and with a game in hand on most of the other basement sides.
Lions boss Donachie was forced to re-shuffle his pack in the absence of left-back Danny Senda, with Mo Ross making his first start since 1st December.
Millwall could have taken an eighth-minute lead when Byfield - clearly relishing the match against his former club - was denied by a brilliant parry from visiting keeper Dean Brill.
Within 60 seconds it was Gillingham who had edged into the lead courtesy of a crisp finish from Gary Mulligan after Bas Savage had flicked on a long throw.
The Lions replaced injured winger Chris Hackett with Chris Zebroski on 24 minutes, and it was the young substitute who fired an unlikely leveller, smashing home at the far post following good work by Byfield and Neal Ardley.
Matt Jarvis, who has been the target of much transfer speculation, fired wide when he arguably should have done better just before the break.
Few in the 10,055 Den crowd could have expected the second-half to have turned out as it did. Not least the Gillingham supporters, who watched in disbelief as their team produced a calamitous collapse as they simply caved in.
The introduction of Ben May for Poul Hubertz proved an inspired change at the break, and within three minutes of the restart Millwall had forged a 2-1 lead. Byfield was sent sprawling by Duncan Jupp and the former Gills striker stepped up to calmly convert the resulting spot-kick.
It took a world-class save by Lenny Pidgeley to deny Andrew Crofts, with the keeper somehow getting his fingers to the latter's 30-yard blockbuster and tipping the ball over.
Marvin Williams then cleared off the line as The Lions looked a bit jittery, but those nerves were settled when Byfield added his second - and Millwall's third - 11 minutes from time after latching onto May's flick and firing past Brill.
Byfield wrapped up the win when he completed his hat-trick on 85 minutes, finishing clinically after again taking advantage of a ponderously slow Gillingham rearguard.
In fact such was Millwall's domination that further goals could have been added by May - who was denied by the woodwork - and then Marvin Elliott, with the midfielder's drive curling inches wide in injury-time.

Millwall: Pidgeley, Dunne, Ross, Elliott, Robinson, Shaw, Williams, Ardley, Byfield, Hubertz (May, 46mins), Hackett (Zebroski, 24mins). Subs not used: Day, Phillips, Fuseini.
Bookings: Zebroski (37mins), Williams (71mins)
Scorers: Zebroski (37mins), Byfield (49mins, pen, 79mins, 85mins)

Gillingham: Brill, Jupp (Nsungu, 84mins), Easton, Sancho, Crofts, Chorley, Savage, Flynn, Bentley (Spiller, 76mins), Mulligan, Jarvis. Subs not used: Pouton, Jackman, Johnson.
Booking: Easton (84mins)
Scorer: Mulligan (9mins)

Ref: Keith Stroud
Att: 10,055 (Away 1,182)