Josh Tongue claimed 4-42 while Ollie Robinson and Jofra Archer shared six wickets as Pakistan's shake-up since going 2-0 down in the three-match series had minimal impact on their miserable fortunes.
Gay and Brook then made fifties on Wednesday's opening day in Birmingham to lift England to 4-156 at stumps, leading by 23 with the promise of plenty more to come, despite captain Joe Root being out lbw for the 10th time in 13 innings.
While the floodlights were on all day amid heavy cloud cover for large spells, the pitch was relatively blameless. Still, Pakistan, with four changes to their side since Lord's, were cowed by England's on-song seamers and look in severe danger of being whitewashed in the series.
Robinson was magnificent, conceding just 15 runs in 11 overs, as he went from 99 to 102 Test wickets, while his average of 19.1 is the best post-war of any bowler with more than 100 dismissals.
It was unsurprising he made the breakthrough in his second over after Root opted to field and exploit the overheads.
Saim Ayub, brought in for his first first-class appearance since October 2025, was Robinson's 100th Test victim.
Ayub, one of the raft of new arrivals when Pakistan banished seven players and two coaches last week, was dismissed for a 10-ball duck after missing a lavish drive off Robinson coming round the wicket.
Azan Awais and Shan Masood both got into awkward positions from innocuous Archer deliveries and succeeded only in bunting tame catches into the leg-side, while Abdullah Shafique edged Robinson low to Dan Lawrence at gully.
When Tongue struck in his first over, obliterating the top of off-stump to send Pakistan captain Babar Azam packing, it meant all of the tourists' top-five had all been dismissed for single figures – the first time that has happened in the first innings of a Test in England.
Saud Shakeel showed a measure of resistance after being given a reprieve on seven, with Ben Duckett dropping his second catch of the morning at first slip off the luckless Gus Atkinson.
Tongue, though, saw off debutants Ghazi Ghori and Mohammad Imran, while quick-bowling debutant Razaullah nicked Robinson first ball after lunch to Duckett.
Shakeel's defiance ended on 43 after a beauty from Tongue uprooted his off-stump, before Mohammad Abbas and Mohammad Ali put on 26 for the last wicket, the highest partnership of Pakistan's miserable innings that ended in 33.5 overs.
Ali made the breakthrough when England batted, Duckett's wild slash when he was on 11 taking a thick edge and flying to second slip.
Jordan Cox was pinned lbw by Abbas while England were 3-39 just after tea when 21-year-old Razaullah, operating at around 90mph, won an lbw decision in his first over to get rid of Root.
But Gay, with his third Test half-century, and Brook took the sting out of Pakistan, as they compiled a 104-run stand.
Though Gay dragged Razaullah on to his stumps for 60, Brook moved past 1000 Test runs against Pakistan in just 14 innings as he went to stumps unbeaten on 52.
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