Proponent of Gaia/Galaxia here.

I should issue an apology in advance if I end up rambling uneducated off-topic nonsense because I just finished reading the 5 books yesterday, which has me feeling passionate. Also, I've never read other Asimov's work before – sorry.
Recently, I am also under the influence of the Netflix show Sense8, which deals with a separate human species that exists within our modern society, unbeknownst to everyone or, most of the time, even to themselves. Akin to what Bliss says about Gaia, those people are able to share emotions, skills, sensations, and even conversations, regardless of the distance between them (the difference being that they come in clusters of 8. Hence, the number 8 in the title), which makes them extremely formidable on one hand, and on the other, makes them be in touch with their own humanity in ways that can seldom be as profound and sincere in non-sensates. It could also be said that people in those clusters make up an entity, but in an indirect way, through their actions, which serve to help or defend each other from dangers. All in all, a different concept, but the show had the luxury of placing that relationship between connected people in its forefront, while F&E had, literally, an entire universe of things to deal with aside from that. Thus, in the books, we are forced to trust in Bliss’s words when she says that there is no greater privilege than to be a part of something as all-encompassing as Gaia. The show lets us experience the beauty of such a connection that tears down all the boundaries set by temporary constructs such as ideologies or prejudice - a real, authentic existence.
(Onto topic) This is why I agree with this opinion earlier in the thread:
Regarding the last statement of Trevize and also his decision on choosing Galaxia. I think it refered to the fact, that even if the alien threat was not yet present, the humanity as a whole is in danger of loosing it's humanity(or the part that made it human), and the solarians (represented trough the solarian child) were the example of that fact. So maybe Trevize made his decision in fear of the alienation inside humanity itself.
Partially biased because of the show Sense8, I believe that being a part of a Gaia-like entity doesn't necessarily mean losing what makes us essentially human. "What makes us human?" is a question in itself. Gaia has respect towards beliefs of individuals, which is why it surrendered its fate to Trevize. Making a leap of faith is quite a human thing to do, imo. Also, Gaia has expressed approval of Seldon's plan, which implies that the Second Empire will be established, only this time, with potential for its eternal longevity.
If I distance myself from defending Gaia for a second, I start seeing that Asimov maybe wanted to ingrain doubt in the DNA of these books, even with the existence of mechanisms like psychohistory (that is at some points implied/feared to be theoretically applicable on individual level even). You can be a proponent of Gaia and a proponent of individualism. Both have merits and deductors, as percieved by us, the readers. For what existence is completely certain, anyway? This is why, I think, Asimov decided to end on such a disquieting note, and why people should be giving F&E more credit than they are.
Even with attempted ambiguity, Asimov, through Trevize's judgement, gives us his opinion.