The League One leaders head to the Priestfield Stadium on Boxing Day to face the first side to beat them in the league this season.
Steve Lovell's men inflicted a 2-0 defeat at Fratton Park in October, consisting of a stunning Tom Eaves strike.
It ended the Blues' 11-match unbeaten start to the league campaign '“Â and only Charlton have since conquered them.
The Gills are presently 19thÂ following Saturday's 2-0 loss at Charlton, their first in five fixtures.
AndÂ Jackett's men must avoid becoming a prized double for the hosts.
Pompey's boss said: '˜We played pretty well that day against Gillingham, in all the games so far this season we have been in those games.
'˜It's a separate match, going there Boxing Day is a tough one, no doubt about that.
'˜When we met before we couldn't get the goals, weÂ couldn't get the breakthrough. They scored twiceÂ in the first half, one of which was a great volley by Eaves.
'˜Now we go to their place and we know, as we go through the second half of the season , that we must apply ourselves, dig deep and work hard for every single point we earn.'
The Blues also head to Kent with a club-record 14-match unbeaten away run in all competitions.
The last loss on their travels came at Bury on April 28, when they succumbed to a 1-0 reverseÂ in the penultimate match of the 2017-18 campaign.
Since then, Pompey have fulfilled away fixtures in League One, the FA Cup and the Checkatrade Trophy without being defeated.
Saturday's crucial win over Sunderland ensures they have now collected 51 points at the half-way mark of the campaign.
And they will be bidding to extend that advantage at Gillingham.
Jackett added: '˜I look overall at the points total and where it hopefully can take you.
'˜For us, you have to make sure you concentrate on one game at a time, don't look at the big picture too much, and realise we have a good chance.
'˜Our preparation and professionalism will give us every chance and opportunity to do our best, which is what we are trying to do.'