It was revealed in February that Hulu is working on a new series in which the 47-year-old actor will reprise her title role as Buffy Summers but won't be the main focus of the program.
Gellar, who will also serve as executive producer, warned fans they need to be "patient" when it comes to seeing the project, but said work is already under way.
"We've set up our offices, which means things are progressing probably faster than I'd anticipated. But it's going to be a process, I think," she told People magazine.
"To do it the way it needs to be done, everyone has to be patient with all of us, because this isn't something that (will just happen)."
The pilot episode will be directed and executive produced by The Eternals director Chloe Zhao, and Nora and Lila Zuckerman are attached as showrunners, writers, and executive producers.
Gellar - who will also be joined by the original series' executive producers Gail Berman, Fran Kuzui and Kaz Kuzu - is "excited" about the idea and the group behind the reboot.
"I pinch myself every day when I get into these rooms, and I'm looking at Gail Berman, the Zuckermans and Chloé Zhao. It's an incredible group," she said.
"There was no world where I thought Chloé Zhao was going to call me and partner with me, and ask my advice on things and my opinion — especially because she hasn't done television.
"She'll call me, and she'll say, 'Well, I have to defer to you,' and I'm like, 'Wait, what?' Those aren't words I ever thought I would hear."
Gellar - who has children Charlotte, 15, and Rocky, 12, with husband Freddie Prinze Jr - is proud the original Buffy, which ran from 1997 to 2003, still has a strong fanbase today.
"As an actor, you just want to do work that stands the test of time, that people still enjoy, not just when it comes out, but years later, and it still is impactful for them," she said.
"So the fact that this show is still so impactful to the people that watched it the first time along with me, to young kids today that are just discovering it and feel like it still speaks to them, being able to bring more people to her, it's the greatest gift an actor could ever ask for. Truly."
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