PORT ST. LUCIE, Fla. — The Mets came closest on Tuesday to publicly declaring what has been obvious for much of spring training. Robert Gsellman will be on the roster come Opening Day.

While manager Terry Collins left some wiggle room about Gsellman’s role — at least initially — he essentially ruled out sending the righthander to the minors. So, the only suspense surrounding Gsellman this spring revolved not around his arm, but his bat.

And even that was put to an end in Tuesday’s 3-3 tie with the Cardinals. For the first time, Gsellman swung the bat in a Grapefruit League game. He grounded out twice, though he’s no longer confined to bunting as he was last season, thanks to an injury to his non-throwing shoulder that eventually required surgery.

“Yeah, it’s fun,” Gsellman said. “Bunting was boring.”

The swings were meek, but they were swings all the same, bringing to an end to what became a longstanding oddity.

“It felt good,” Gsellman said. “My timing’s a little off, but it felt good to swing and I felt [like] part of the game.”

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As for his primary purpose, Gsellman only solidified his claim to a spot on the roster. In six innings, he surrendered all three of his runs on perhaps his only a mistake, an opposite-field homer given up to Matt Adams.

The curveball was supposed to bounce in the dirt. Gsellman let it hang in the zone. Adams pounced. But the righty struck out six, walked one, and wrapped up his spring training with a 2.31 ERA.

“Very pleased,” Gsellman said. “There’s still more to come and I’m starting to feel better than the beginning of the spring.”

Gsellman, 23, bailed out the Mets late last season. In eight appearances, seven of them starts, he went 4-2 with a 2.42 ERA. His 44 2/3 innings were pressure packed, with the Mets roaring back from a slew of injuries to reach the wild-card game.

“He’s really had a good spring and throwing the ball very well,” Collins said. “Picked up where he left off.”

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Indeed, Gsellman proved he was no fluke, blending plenty of velocity with a nasty two-seamer to keep batted balls on the ground. Rival scouts believe he’s already a quality mid-rotation starter.

“There are guys who just have a feel for when they’re on the mound and a demeanor when they pitch, and I think Robert’s learned a lot from his experience last year,” Collins said. “He sees the older guys, and the way they prepare this spring. I think he’s grown up in his preparation and how he goes about things.”

Gsellman’s next assignment will depend on how the Mets juggle their rotation to begin the season. At the center of their decision is lefthander Steven Matz.

If he’s healthy by Opening Day, Gsellman likely would be the Mets’ fifth starter. And because that turn in the rotation doesn’t come up until April 9, he could be stashed in the bullpen temporarily as a long reliever.

If Matz isn’t healthy, Gsellman likely lines up behind the Mets’ established trio of Noah Syndergaard, Jacob deGrom and Matt Harvey.

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In either scenario, Gsellman is seemingly bound for New York, where he’ll have the chance to further establish his place in a loaded starting rotation.

“I’m just getting mature, a lot stronger, a lot more wise,” he said. “I’m feeling way better than I did last year for sure. Now I can swing and add to my game — a little bit.”