Northampton were cruising with a 21-6 lead when replacement scrum-half Alex Day, who was part of the England Under-20 side that won the Junior World Championship for the first time in the summer, came on for his debut and lit up the crowd with two tries in the final eight minutes to make it 33-6.
Ken Pisi's first-half double plus 11 points from the boot of Glenn Dickson and two from Olver completed the rout.
King said: "It was a great performance, the forwards were outstanding and the replacements that came on did really well.
"It is cracking for young guys to play in front of 12,000 fans. The young lads are training every day with the established players, they deserve their chance and hopefully they will crack on now.
"Alex finished off the job that Glynny (Ryan Glynn) started and it was great to see him get over for the bonus point try.
"I have got high hopes for all the young players, they have just got to work - there is no secret.
"Alex has spent a bit of time at Moseley, which is important for his development - managing himself for a whole league season. He is a strong boy and he is having a good battle with Glynny - this competition is fantastic for those two players."
Gloucester forwards coach Andrew Stanley was left to rue his scrum conceding numerous penalties and the sin-binning of Matt Cox that saw them ship 14 points in 10 minutes, with their only points being two penalties from captain Freddie Burns.
"If we had been a bit more clinical with the chances we did create things may have been different but that wasn't the case and it got tough in that second half," said Stanley.
"We have got to work very hard to change people's perceptions about our scrum."