Is Goldeneye a reboot?

I don't think it was necessarily intended to be one -- and the presence of Desmond Llewellyn as Q argues against it being one -- but I think you can choose to regard it as one if you like, and the film doesn't give you definite indicators that it isn't.

From an age standpoint, Brosnan's Bond is certainly too young to have been working for MI6 in 1962 when Dr. No came out (and so was Dalton's for that matter). Then again, Bond is kind of like Batman in DC Comics -- eternally about age 35, ever since he was introduced 85 years ago in 1939, and you just ignore that fact and suspend disbelief in order to keep enjoying the stories. You're not really meant to overthink it and start quibbling about timelines and specific years and ages and so on.

As another comment said, it's not intended to be a reboot. It's intended to show James Bond can adapt to the world of the 1990s. Those behind the series had to deal with the end of the Cold War, the disappointing reception of License to Kill, and betting that audiences would like Brosnan as Bond.

"there's also no evidence that it's in the same universe as the previous movies."

Like the poster above said, there's Q.

Goldeneye is almost a soft reboot considering it was meant to jumpstart the franchise that had been on life support for the past 5-6 years. It was the first Bond film not produced by Albert Broccoli, without longtime series cinematographer Alec Mills, and various other cast and crew not returning. So everything felt new and fresh.

M makes a comment that Bond is “a relic of the Cold War”. But the film also seems to carry over from the Dalton era, with the opening Dam dive scene clearly meant to feel like Dalton, until it’s revealed to be Brosnan.