The actor explained that he is on an "episode-to-episode basis" and does not know whether he will be asked to film more scenes for the Fox show.
"It's all up in the air right now," Gustin told TV Guide. "I have no idea what's next for him, to be honest."
Rumours have previously suggested that Gustin could be a series regular in Glee's fourth season but he explained that he has not heard anything about that.
"They have an option on me," he said. "Basically, if I am to be back next year, that's most likely what would happen. But they haven't asked me back yet, and I don't know if that's going to be the case."
However, Gustin suggested that there are "plenty of possibilities" for Sebastian storylines after the Warbler re-evaluated his behaviour following Karofsky's attempted suicide in the latest episode.
"What happened to Karofsky (Max Adler) hit very close to home for him," he said. "It shocked him into reality, I think. I was surprised it happened so soon. I don't know if the snarkiness is gone now or if he's a nice guy, but seeing a different side of him is exciting."
Glee will return with new episodes on April 10 on Fox in the US. The show resumes its third season in the UK on March 1 on Sky1.