The 23-year-old Gsellman struck out six in six innings Tuesday against St. Louis in a late audition to make the team.
Asked if he expected Gsellman would begin the season at Triple-A Las Vegas, Mets manager Terry Collins responded, "I would not."
"He's really had a good spring and thrown the ball really well. He's picked up where he left off," Collins said.
Collins stated a decision had not been made on Seth Lugo, who was strong -- as was Gsellman -- late last season as New York chased down a wild-card berth.
Lugo has been part of the equation from the start, often included in the same sentence or context with Gsellman. They've both been mentioned when talk turns to fifth starter Zack Wheeler, who had Tommy John surgery in 2015 and is still working his way back into form.
"I think it's too soon with him," Collins said of Lugo. "We've got to have somebody stretched out who can come up and step in (as a starter). The first seven days of the season we're OK. But once we get to the point where you've got to use that fifth (starter), we've got to make sure we have protection in the bullpen."
Gsellman could be used in the bullpen as the long man, too, Collins added.
Following the split-squad starts of Lugo and Wheeler the day before, Gsellman gave up three runs and five hits in a 3-3 tie with the Cardinals. Gsellman said he didn't put any added pressure on himself to impress anyone.
"I didn't take it that way. I'm going to pitch again in five days wherever I go," he said.
Also, left-hander Steven Matz appears to be on the fast track after Sunday's announcement that he was being slowed by elbow tenderness. The Mets' projected No. 4 starter long-tossed on flat ground on Monday and returned to the mound in a bullpen session monitored by pitching coach Dan Warthen on Tuesday.
The Mets are not exasperated at the oft-injured Matz's current ailment -- the team wants him to pitch on Thursday -- but Collins said they will continue to try and get a feel for their lone southpaw starter.
"We've got to get him on the mound to where he can really throw some pitches and see where he's at," Collins said. "We've certainly got to see how he comes out of it. In our situation, it's really tough to have that guy who's not able to give you any innings.
"Because of the workload early in the season, you've got to make sure you don't kill your bullpen too early," he said.
The Mets have exhibition games on Thursday and Friday, giving the staff more time to make decisions before the final roster is set over the weekend.
"As much as you'd like to say we're going to only have five starters, we know that's not the reality anymore in today's game," Collins said. "These decisions aren't made on the immediate but on the long term."
The Mets are still waiting to hear on a possible suspension for reliever Jeurys Familia, who met with Commissioner Rob Manfred on Monday in New York to discuss the closer's domestic violence incident, which was dismissed in court. Familia was back in camp on Tuesday. . Collins said the MRI results on CF Juan Lagares's left oblique injury were "better than we hoped." He is day-to-day, and Collins said it is possible he could be in New York next week when the season opens. . OF Brandon Nimmo (hamstring) said he is putting in all the work to get back to full health, except for running sprints, after getting hurt in a World Baseball Classic game playing for Italy on March 12. Nimmo, 24, was diagnosed with a Grade 1 right strain. . The team re-assigned RHP Paul Sewald and optioned C Kevin Plawecki to the minors.