It was fly-half Shane Drahm who inflicted most of the damage on the visitors, contributing 14 points with the boot to add to first half tries from prop Chris Budgen and wing Jon Sleightholme.
A curiously one-sided affair, Gloucester were trailing by 27-0 at the one hour mark with seemingly no chance of overhauling their opponents when they suddenly sparked into life.
Admittedly they were helped by the dismissal of Northampton No.9 Mark Robinson for punching just before the break and the yellow-carding of replacement scrum-half Ian Tucker late in the second half, but gave the Saints something of a scare.
The revival was started by flanker Andy Hazell – a recipient of the yellow card himself in the first half for a late challenge on Drahm.
He took the direct route to the tryline in the 60th minute to give Gloucester their first points, Henry Paul supplying the difficult conversion.
Then came a stunning late cameo appearance by replacement Forrester, who made a major impact when he joined the fray with less than 10 minutes of normal time left on the clock.
The rangy backrower scored two tries in the 74th and 78th minutes to make the final scoreline far more respectable for Gloucester.
But it was Northampton who had the last word in the match, Bruce Reihana kicking a penalty in injury-time to seal the result.
For Northampton: Tries: Budgen, SleightholmeCons: Drahm Pens: Drahm 4, Reihana 2
For Gloucester:Tries: Forrester 2, HazellCon: Paul