Kenny Jackett pointed the finger at Pompey’s defending for the second-half capitulation which leaves them winless at Fratton Park in 2018.
Gillingham fought back to claim a 3-1 victory on Saturday – their first PO4 win in the club’s history.
It was tough to stomach for the Fratton faithful, who watched their side enter the interval with a 1-0 advantage.
That was a much-improved opening 45 minutes, the lead registered by Jamal Lowe.
Yet the Gills struck three times in the second half, with skipper Lee Martin netting twice.
It means Pompey have claimed just two points from a possible 18 at home since December 30.
We were doing some very good things and played very well in the first half, but it needs to be a 90-minute performanceKenny Jackett
Jackett said: ‘We possibly could have been further ahead in the first half, although I always felt Gillingham were going to come back strongly.
‘The substitution with Wilkinson coming on had an instant impact.
‘From our point of view, we shouldn’t let him get to the ball first and then be able to work it from the right channel back onto his left foot across us and into a shooting position.
‘We’ve let him get into a very good position.
‘He’s hooked it over one centre-back, run across the other and then hit it.
‘We can also look at other situations, such as Martin with the goals he scored.
‘I understand that perhaps one of them was a cross and it has gone in at the far post, but we’ve got three good centre-halves who should be able to defend that situation.
‘We were doing some very good things and played very well in the first half, but it needs to be a 90-minute performance.
‘We had to realise at some point Gillingham were going to come back and put some pressure on us.
‘During that period, we conceded three goals and gave them the opportunity to battle back in the game when we were on top.’
Pompey were booed at the final whistle following their latest Fratton Park failure in 2018.
It has been two months and 11 days since they last tasted victory on home soil against Northampton.
That period consists of six League One games and one Checkatrade Trophy outing – and includes five defeats.
Jackett added: ‘We talked at the break about Gillingham coming back and us making sure we were strong enough to withstand.
‘I spoke about wanting to start strong, going towards our own support, and seeing if we could build on the half and not replace it.
‘In the second half we had Dion Donohue’s chance on the edge of the box where he couldn’t quite beat the goalkeeper, while Connor Ronan was unlucky with a free-kick where he has hit the post, which surprised the keeper.
‘We obviously have players out and they’ll be periods in the game where we need to sustain that pressure and make sure they don’t score.
‘Ultimately, that has cost us.’