Goals from Marvin Elliot and Barry Cogan after the break sealed the victory after a Mark Phillips header had levelled the scores just before half-time.
But the Lions didn't have it all their own way. Depsite making a lively start, it was the Conference side that took the lead on the half-hour mark when Andrew Drury drilled the ball past 'keeper Tony Warner. Millwall piled on the pressure with Neil Harris going close on two occasions before Phillips equalised on the stroke of half-time.
The Lions included Argentinian trialist Meldondo Duarte in their starting line-up together with captain Stuart Nethercott, who is rumoured to be considering his future at the club following comments by chairman Theo Paphitis. But manager Mark McGhee rang the changes during the interval and sent out a completely different team.
Millwall finally took the lead 10 minutes after the restart when Kevin Braniff played in winger Cogan, who rounded the keeper to slot home. Braniff departed the field shortly afterwards with a minor injury and it was his replacement, Marvin Elliot who made the game safe nine minutes before time.
Gravesend & Northfleet: Lee Turner, Lee Shearer, Francis Duku, Robin Trott, Justin Skinner, Rob Owen, Steve McKimm, Andrew Drury, Scott Goodwin, James Pinnock, James Bent.
Millwall first half: Tony Warner, Matt Lawrence, Mark Phillips, Stuart Nethercott, Meldondo Duarte (trialist), Steven Reid, Tim Cahill, Andy Roberts, Peter Sweeney, Neil Harris, Paul Ifill.
Millwall second half: Willy Gueret, Tim Clancy, Darren Ward, Matt Rees, Robbie Ryan, Dave Livermore, Barry Cogan, Dennis Wise, Ronnie Bull, Kevin Braniff (Marvin Elliott, 56mins), Mark Quigley.