Beyond that, here this really is an after-the-fact situation (since she is already pregnant), so there is no need to get into the question of definite versus uncertain. Even if this is a case of definite versus definite, it is still clear that it is after the fact.

Meaning, according to this explanation, the question is whether the child is fit.
If she claims it is from the betrothed and the betrothed says it is from him, we accept the claim.
Abaye objects: would Rabban Gamliel always say that the woman is believed, after Rav Yosef’s claim that we accept it both because the husband admits it and because of Rabban Gamliel?
I’m not sure this is the after-the-fact case being discussed here. The fact that the woman is already pregnant does not strengthen the claim that it is from the betrothed. I actually understand the after-the-fact case as referring to after the marriage — if she claims, “I was injured accidentally” or “I was raped after I was betrothed,” that is after the fact. But if the woman makes the claim beforehand, then she is forbidden to a kohen even according to Rabban Gamliel.
In short,
why, if she is pregnant and the betrothed claims it is not from him, does that render the child fit? It still looks like definite versus definite.

Because the whole point of Rabban Gamliel’s disagreement is about her credibility regarding herself. But regarding the child, that is a different matter. For example, because here the testimony is not about herself but about someone else. In any case, that is what Tosafot writes there.

I looked for a discussion of definite versus uncertain and didn’t find one. Still, I’d like to ask the Rabbi:
What is the reasoning that a definite claim against an uncertain one helps even when there is established possession of money? (There is a dispute in the Talmud in Ketubot 12b whether a definite claim is stronger or established possession of money is stronger, and this also appears in the case of “you have one maneh of mine in your possession,” and elsewhere.) Imagine it: a person claims that someone owes him money, and the other says he does not remember — would we obligate him just because of that, even though he is in possession of the money? And I heard that some suggest that because he says “perhaps,” he undermines his own presumption of possession, but that is hard to say regarding a loan, for example, where we are not talking about some specific item of money that he is holding and thereby undermining his possession because he claims uncertainty.

I’d be glad to hear the Rabbi’s opinion.

As I recall, there is a lecture about this in Bava Batra. You can look there.
In my opinion, it is not an undermining of established possession, but one of two things: 1. In a weak uncertainty claim, that itself is evidence against him, and evidence, like a presumption, can extract money from someone in possession of it (for example, the presumption that a person does not pay a debt before its due date). 2. In a strong uncertainty claim, there is no evidence against him, but one can still say that a presumption helps only if there is also a claim alongside it. If you are not making a claim, what use are evidence and presumptions to you? The evidence and presumptions operate only after there are two litigants presenting their claims before a religious court. If you do not make a claim, the evidence and presumptions have no meaning. This may depend on the question whether an uncertainty claim is considered a claim or not (I recall that Kovetz Shiurim discusses this).

Hello to our master, may he live long, and thank you for the answer.
Unfortunately I did not find the lecture in Bava Batra; it may be under a different title.
Regarding your first answer — you are relying on Tosafot’s interpretation, that they explained the passage as saying that definite versus uncertain applies specifically when the definite claim is strong and the uncertain claim is weak. But Tosafot’s words are not really compelled by the passage to mean that definite versus uncertain refers דווקא to a strong definite claim and a weak uncertain claim.
Regarding the second answer — if a person is in possession of land and one claims that it is his land and the other says “I don’t know,” then indeed that is true. But in the case of “you have one maneh of mine in your possession,” when he is in possession of money and we are not speaking of some specific money item that exists before us, it is very hard to say such a rationale — that because he does not remember whether he owes him or not, we obligate him. Just imagine someone comes up to you on the street and asks: “Do you remember that I lent you money? You still haven’t returned it.” And you claim that you do not remember whether you returned it or not. Maybe it would be proper to ask this of him in the laws of Heaven, but to obligate him in court?
Thank you!

That is exactly what I said. In my opinion, it is indeed reasonable. First, there is a presumption that a person does not make a claim unless he has something to base it on. And a person has a presumption of validity so long as he has not been disqualified. Beyond that, the presumption does not stand for you without a claim. I do not see any difficulty in this. Yes, if I do not remember, then he will win the case. Does it seem reasonable to you that someone who borrowed from me could tell a convenient lie by claiming he does not remember and thereby win? By the way, the case you described is a weak uncertainty claim.