Sure, they might have the skills to learn about a new species, but isn't it self-evident that someone who's already learned how to take care of them, someone who's been working with them for years already, is at a major advantage?
An "advantage", sure. The end-all, be-all, authority on all thngs cetacean? Doubtful. We at the very least know that Humpback Whales don't survive until the 23c, but other orca species could have who'd provide a place to draw from on how to treat George and Gracie.
Given how urgent and difficult it would be to keep a single breeding pair of whales alive in an alien and hostile environment, there may not be time for a learning curve.
Alien hostile environment? They were returned to the very same ocean they just left! Hell they were returned to a ocean that's likely cleaner and more hospitable as well as now being on a planet populated by a species that appreciates the sentience/sapience of whales.
They'd need to be taken care of and carefully monitored and protected from day one. So someone who already knows about them is of obvious and immense value, and it would be insanely irresponsible not to take advantage of her expertise.
Again, I don't doubt Gillian's "expertise" would have proven to be somewhat useful but I'd think the sensibilities of the future as well as the centuries of learning would push quite hard against Gillian's schooling.
There's things about surviving ocean species that the 23c scientists know that are beyond Gillian's understanding.
That doesn't make any sense. Gillian was a PhD in cetacean biology and a passionate advocate for conservation. How could she possibly be part of the system of commercial whale hunting? Especially since humpback whale hunting was outlawed when she was fifteen years old (assuming she was the same age as her portrayer).
I don't think the 23c people are going to be so quick to separate Gillian from the rest of the 20c people as a while. We've been shown time and time again in Trek (more notably in TNG) how much 20c humans are looked
down on In this very movie McCoy grumbles about wondering how humans survived past the 21st century and has no faith in the medicine of the time. Yet at the same time we know that there's people on Earth who'll have meaningful (positive) impact on the future McCoy enjoys. In the 1980s the very beginnings of medical research with DNA and such is starting up that's doubtlessly
endlessly useful to McCoy's job every single day. Still doesn't mean he's going to accept 20th century medicine
I think Gillian will be looked at the same way, sure, she's "one of the 'good' ones, but she's still "one of them."
Don't you think that, say, historians who studied Zefram Cochrane would relish the opportunity to talk to someone like Lily Sloane who actually knew him personally and worked alongside him for years? Do you really think they'd consider firsthand experience with the subject of their research to be worthless, or that they'd look down on her firsthand knowledge as somehow inferior to their purely academic, indirect knowledge?
You bring up a good point on the Cochrane thing. Because in First Contact we're very nicely shown the difference between hour our heroes, founders and people we idolize are in history books and how they really were. We don't know how the 24c thinks of Lilly but we are shown that Cochrane left much to be expected.
In something of a conclusion:
I'd think the most they would do with Gillian is "pick her brain" to get some basic idea of what to do and what to expect. But that's about it. There's the transporter trace on the Klingon ship which can provide a great deal of information about the species on a quantum level, they have telepathic species (like Vulcans) who can communicate
directly with George and Gracie and depending how much of grasp humans have when it comes with dealing with other cetacean species there may be other ways to communicate with them. Gillian is no doubt a good source of information, but she's not the
only one. The Federation has much, much more at their disposal to get information they need and they have centuries of experience when it comes to learning about species no one knows anything about. (Given that that's pretty much their entire goal.)