On an unusually sweltering October afternoon at Oakwell, Gray reached his milestone from the penalty spot shortly before half-time. And 10 minutes after the break he began his journey towards 200 goals with a close-range tap-in.
The result sends Coventry into the relegation places and leaves Andy Thorn's side without an away win so far this term.
Roy O'Donovan spurned an early opportunity to put the Sky Blues ahead, scuffing his shot into side netting when it was easier to score. Barnsley followed suit, Ricardo Vaz Te heading wildly over the crossbar following a pinpoint cross from Danny Drinkwater.
Gary McSheffrey then drove forward and took aim from distance, but his swerving effort sailed wide of Luke Steele's goal.
Vaz Te was first to register a shot on target, his free-kick following Conor Thomas' foul on Jacob Butterfield stinging the palms of Joe Murphy in the Coventry goal.
Butterfield was soon kicking himself after wasting a clear chance to put the Tykes ahead, his left-footed shot devoid of the required power and accuracy and bobbling left of the post.
Lukas Jutkiewicz, a constant threat to the hosts' backline with his pace and trickery, manoeuvred to blast at Steele five minutes before the break, but the former Manchester United goalkeeper was equal to the attempt.
Referee Geoff Eltringham showed no hesitation in the 42nd minute when Chris Hussey handled in the penalty area. Gray stepped up to send his spot-kick into the bottom right corner and register a century of goals in professional football.
The Tykes might have doubled the lead immediately after the restart, Murphy parrying a fierce free-kick from Drinkwater following Martin Cranie's foul on Gray. Butterfield lofted in an inswinging corner but Gray's header from the centre of the box was off target.
But the frustration would not last for long. With 55 minutes played, Jim O'Brien shimmied his way behind the Coventry backline to supply Gray with an inch-perfect cross, which he gratefully tucked beyond the diving Murphy.
And it was almost three shortly afterwards when O'Brien wiggled into space on the edge of the penalty area and fizzed a shot into Murphy's gloves.
The visitors began to wilt in the Yorkshire heat, and could not summon up a shot on target as the game entered its final stages. Jutkiewicz miscued a left-footed drive which failed to threaten the Barnsley goal and David Bell's solid drive drifted beyond Steele's left-hand upright.
Jutkiewicz leapt to meet a Conor Thomas cross with only three minutes on the clock, but his header bounced high above the home goal and into the crowd.