The former Leeds and Anderlecht striker was the weekend hero as his double helped rescue Rangers from defeat to Motherwell.
And he followed that up with the opener at McDiarmid Park as the Light Blues cruised to their 25th win in all competitions this season.
Glen Kamara added a well crafted second goal before Ianis Hagi marked his first league start for two months with the third just after half-time as Gerrard’s side maintained their 16-point lead at the top of the Premiership after hitting the half-way point.
A miserable night for third-bottom Saints, who are now six games without a win, was compounded in the 83rd minute as substitute Michael O’Halloran was sent off just five minutes after coming on as he picked up two quick bookings.
Gerrard promoted Joe Aribo and Hagi from the bench as he rewarded the part they played in turning around Rangers’ weekend clash with Well, but Alfredo Morelos started among the substitutes on his return from suspension.
In days gone by, the Colombian could always expect to come straight back in to the team no matter his latest indiscretion, such was their reliance upon him.
But times have changed now, with Roofe on his way to establishing himself as the main man at Ibrox.
However, Rangers did almost get themselves into trouble early. There was no sign of Saints copying Motherwell’s ultra-defensive 5-5-0 set-up from the weekend as they instead set out to press the leaders high.
Rangers were almost caught out a couple of times with some square balls, including one example in just the third minute when Kamara fired the ball straight to David Wotherspoon, forcing Connor Goldson and Filip Helander to intervene to snuff out the danger.
Rangers had been forced to earn the right to play but having gone toe-to-toe with Saints, they moved up a gear and grabbed the opener in the 24th minute.
Ryan Kent beat Craig Conway to nudge on James Tavernier’s loose pass. While Saints complained for a foul, Rangers raced forward with Kamara before the ball was laid off to Hagi. The Romanian fizzed in a low left-foot drive but Zander Clark’s parry laid the ball on a plate for Roofe to tap home.
That was Rangers’ first shot on target and they struck again with their second thanks to a moment of class from Aribo.
The Nigerian picked up possession midway inside the St Johnstone half from a Steven Davis header before measuring a sublime pass through the smallest of gaps in the hosts’ defence, leaving the on-rushing Kamara to nonchalantly prod past Clark.
Saints made a double change at the break as Murray Davidson and Conway were replaced by Stevie May and Ali McCann.
But it made no difference to Gerrard’s men, who struck again inside 90 seconds of the restart.
Aribo was again the instigator, driving forward before sliding in Roofe. Rooney got in a block to keep out the striker’s effort but Clark failed to sort his feet and got in a horrible mess as Hagi’s fierce hit bounced through his ankles before trickling over the line.
May did come close to giving his team a lifeline as he raced past the static Helander onto Kane’s flick-on but there was no way though Allan McGregor, as the keeper pulled off a fantastic one-handed stop.
O’Halloran was introduced after 77 minutes for Danny McNamara but he was soon making his way back down the tunnel as he followed up a soft yellow for a tug on fellow substitute Morelos with another caution 60 seconds later as he clattered into Borna Barisic.
Morelos should have brought his three-month Premiership goal drought to an end with five minutes left as Clark hit his clearance straight to him – but with his confidence levels on the floor, last season’s top scorer tugged badly wide.