Geyer was the star of the show for Queensland on Saturday, as Victoria hit 9-318 declared before the Bulls were 0-15 at stumps on day one of the pink-ball fixture.
Crucially Matt Renshaw survived a tough final half hour under the Gabba lights to be unbeaten on two, in what could be an audition for the day-night second Ashes Test at the same venue.
With questions over Usman Khawaja's fitness and whether Travis Head will remain opener or go back to No.5 after his Perth heroics, Renshaw has a big Sunday ahead of him in Brisbane.
Saturday night's 34-ball survival came with chief selector Tony Dodemaide in attendance, with the opener having already scored two centuries this summer.
Regardless it will be Geyer who left the Gabba happiest after his 4-102 on debut, even if he went at more than six an over.
Struck down by stress fractures in his teens, the 22-year-old paceman needed plenty of resilience after Harper took him down early at the Gabba.
Harper cut the seamer's first two balls for four on his way to 88, in a brutal welcome to first-class cricket for Geyer.
But the seamer responded shortly after, having Marcus Harris well caught at slip for 18.
Geyer also copped some treatment from Matt Short, but recovered to remove Mitchell Perry, Fergus O'Neill and Will Sutherland in the final session.
He would have had a five-wicket haul had Tom Straker not put down a catch at fine leg to dismiss Todd Murphy.
"He's very high energy Sammy," spinner Mitch Swepson said.
"Looked like he would run through a brick wall every time I threw the ball to him. He was ready and raring to go.
"Outstanding for him to get four wickets, he probably deserved five as well. He bowled brilliantly and was that spark for us."
Swepson also took two crucial wickets for the Bulls, getting Harper and Peter Handscomb in quick succession after the pair added 88 for the third wicket.
Harper had been the chief aggressor for Victoria, bringing up his 50 in 55 balls after a series of cuts, late cuts and a big six over mid wicket off Straker.
But he fell when he cut Swepson straight to backward point, before Handscomb (48) picked out the mid-wicket fielder in Swepson's next over.
Veteran Gurinder Sandhu also took 2-55, and was arguably the Bulls' best bowler with the pressure he built with the ball.
15°C