The 38-year-old ripped through the visitors who were all out for 234 in 70.3 overs and, at the close, the hosts' reply was 65 without loss off 23 overs with skipper Daryl Mitchell unbeaten on 46.
Gloucestershire had no answer to deadly Richardson who has taken 20 wickets in six days after his first four Championship games this season had yielded just six for 282 runs.
It was the third eight-wicket haul of his career - and his second against Gloucestershire - which eclipsed his previous best return of eight for 46 for Warwickshire against Sussex at Edgbaston in 2002.
Elsewhere in Division Two, on the second day of play between Leicestershire and Glamorgan at Grace Road, the home side were in all kinds of trouble at stumps.
Starting the day on 256 for five, Glamorgan were eventually bowled out for 442, with James Allenby ending unbeaten on a brilliant 138.
Graham Wagg offered ample support with 58 and then took one of the 10 Leicestershire wickets that fell for 142 - with Michael Hogan and John Glover equally sharing eight of them.
Plenty of wickets fell at Canterbury too - 11 in total - as Kent knocked over Essex for 212 and then ended the day on 20 for one.
Only a seventh-wicket stand of 88 between James Foster (47) and Graham Napier (58) gave Essex a fighting chance as Darren Stevens (four for 36) shone. It was Napier's fifth half-century in eight innings.
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