It's not clear whether all Time Lords are Gallifreyan, but it is certain that not all Gallifreyans are Time Lords.
In the classic series, the Outsiders in "The Invasion of Time" (and possibly the Shobogans in "The Deadly Assassin") were Time Lords who lived outside the city. It's not even clear whether the "plebian" and middle-class citizens of the Capitol count.
In the new series, in "the Doctor's Daughter", after confirming that Jenny has the same physiology, the Doctor says, "A Time Lord is so much more... A sum of knowledge, a code, a shared history, a shared suffering".
The Virgin novels imply that no one is a Time Lord until they graduate from the Academy, and the Virgin and BBC novels tell us that Gallifreyans from the days before Rassilon, Omega, and the Other aren't considered Time Lords (at one point, I. M. Foreman even says that they're a separate species).
On the other hand, it's a bit odd that the scientific name for Gallifreyans is "Dominus Temporis" (which means "Time Lord"), and there are plenty of occasions where the two terms are used interchangeably.
Meanwhile, we've got conflicting information on whether or not all Time Lords are Gallifreyan. In the BBC webcasts, Ace became a Time Lady despite being human, and in the Big Finish audios there was discussion of "elevating" Leela to the same status as well. And of course on TV, Doctor-Donna and the Meta-Crisis Doctor were both apparently "Human Time Lords".
The status of the Doctor's granddaughter, Susan, is similarly up for debate. At the time Susan was part of the series the Time Lord concept hadn't been conceived yet, and no reference to whether she's Time Lord or not is made when she returned in "The Five Doctors". Another unanswered question is whether all Gallifreyans regenerate. The waters were muddied even further in "A Good Man Goes to War" in which Time Lords are explicitly referenced as a unique species with their own DNA. So the question of whether all Gallifreyans are even the same species has to be asked.