Conor Wilkinson was hailed as the game changer as Gillingham grabbed a maiden win at Fratton Park.
Gills manager Steve Lovell felt the former Pompey loanee was the difference as his side grabbed a 3-1 come-from-behind win.
Wilkinson scored a fine goal three minutes after coming into the fray in the second half, as his side were finally victorious at PO4 after 106 years of trying.
Lee Martin added two excellent long-range finishes as Gillingham put Kenny Jackett’s side to the sword after the break.
Wilkinson spent time on loan with the Blues at the end of the 2015-16 season, which amounted to a 21-minute outing against Northampton on the final day of the league campaign.
Lovell had no doubt about his influence, though, after he replaced top goalscorer Tom Eaves.
He told the club’s webstie: ‘Eaves did what he had to do, Conor (Wilkinson) came on and he was the difference for us. I thought we played some really good stuff.
‘Lee Martin is a tremendous professional – he’s a leader and he inspires the team.
‘He drives us on and there’s no substitute for experience.’
Victory over Pompey continues Gillingham’s impressive form under Lovell. His side were deep in the relegation mire until a major upturn in fortunes under his guidance.
They’ve now lost just two of their past 14 games to take them above Jackett’s side in the table.
Lovell said: ‘Goals change games. The boys have worked hard, they’ve put in a shift and they can play football.
‘They pass it well and they’re capable of beating anyone in this league.’
Lovell reserved praise for the 1,551 travelling Gillingham fans who were one of the biggest away followings of the season at Fratton Park.
He added: ‘It’s a great three points and a really good performance. (It was) great support again – fantastic support.’