Having spent the winter enhancing his reputation with England Lions in the Caribbean, Harris looked in the mood to pick up where he left off last summer, when he registered a career-best 63 wickets.
The 20-year-old took the scalps of opener Ian Cockbain and Chris Taylor and could have had a few more after regularly beating the outside edge of the bat.
As it was, Adam Shantry chipped in with two wickets from the River End, while Jim Allenby dismissed visiting skipper Alex Gidman to leave Gloucestershire struggling at 76 for five when bad light brought play to a premature close.
In a pre-season interview Harris declared himself ready to shoulder the burden for the Glamorgan bowling attack this season.
And he may have a hefty workload to carry over these four days with veteran spinner Robert Croft missing with a hamstring problem, Huw Waters unwell and Graham Wagg leaving the field after bowling just one over because of a 'slight' hamstring strain.
Gloucester resumed this morning 126 runs adrift of Glamorgan's scratchy first-innings total of 202.
The Welsh county were indebted to a 138-run seventh-wicket partnership between Ben Wright (83) and Wagg (58) for hauling themselves out of the mire after another top-order batting collapse.
After watching his side slump to 54 for six inside the first hour, new skipper Alviro Petersen must have wondered whether his decision to bat first under grey Cardiff skies was the best option.
The tone was set by the second ball of the day, when opener Gareth Rees was trapped leg before by Jon Lewis.
Three of Rees's team- mates, including Petersen, duly saw the dreaded digit lifted as Glamorgan's early-season struggles with the bat continued.
The problems were being caused by the swinging ball rather than any demons in the pitch and it could have been far worse for the home side had Ian Saxelby hung onto a routine catch at second slip when Wright was on nine.
Making the most of his let-off, Wright — who was presented with his county cap during the tea interval — began the salvage mission in style, producing plenty of aggressive strokeplay to put on 138 in 22 overs with Wagg until both fell in quick succession to 21-year-old seamer Saxelby.
Wright became one of Saxelby's five victims when he was bowled playing a misjudged pull to a delivery that looked to have kept low. Wagg was guilty of a similar lapse, shouldering arms to a ball that clipped his raised bat then clipped the off stump.