The County dominated their hosts for much of the four-day contest, but ended up on the wrong end of a six-wicket defeat as Gloucestershire chased down 266 to win on the final afternoon.
Starting their second innings with a very healthy 136-run lead, the County batters crumbled as they were quickly reduced to 92 for seven under the floodlights.
They were then polished off for just 129 on Thursday morning, meaning Gloucestershire - who were at one point 153 for nine in reply to the County's first innings score of 327 and facing a follow-on - needed 266 to win, and they got there in 80.4 overs
Miles Hammond hit 94, before an unbeaten 67 from Tom Lace and Graeme van Buuren's 53 not out saw the home side to victory, despite a disciplined performance from the County bowlers.
"I didn't see that coming," head coach Ripley told BBC Northampton.
"I thought we were in control of the game, and I thought we were playing really well.
"I find it hard to be too critical of the players, in that when we came out and batted on Wednesday night, the lights went on.
"I do feel it's hard to say 'that was a poor shot' or 'what were you doing?', and I have to say I think the Gloucestershire bowlers bowled very well.
"I think for three and a half days here at Gloucester we have played some pretty good cricket and been better than the opposition, but we are walking away defeated.
"That session of cricket on Wednesday has cost us the game, and it is disappointing.
"Sometimes you get what you deserve, and I don't think we have got that here."
Northants are due to return to action on Sunday when they host Surrey at the County Ground (10.30am start).
The Londoners' clash that was scheduled for Durham this week was called off due to a Covid-19 case in the Surrey camp.