The Railwaymen's good run of form was brought to a halt at Sixfields as Northampton ran riot after conceding an early goal.
"It wasn't a night to remember but, in a funny way, I was pleased with the players," said Gradi.
"Everything that could go wrong went wrong, apart from the first goal.
"It should have been 2-2 at half time but, for some reason, Ashley Westwood changed his mind with the penalty.
"It was one of those nights when players made mistakes and each one they made we got punished for, but I was pleased with the way they kept going and didn't lose their heads. We continued to try and play good football and get the ball forward but we lacked in the final third."
Northampton look to make further progress towards the play-off places this weekend when struggling Barnet are their visitors.