GP is correct that the argument contains an informal fallacy[1], and people often express this criticism by saying "your argument is tautological," and you are correct that a tautology is a statement which is always true [2].
> GP is correct that the argument contains an informal fallacy
The GP didn't say that.
As for which informal fallacy the GGP's argument contained, I would say it was more like a false dilemma. And the GGP did say "almost tautological", which implicitly recognizes that there might be other possibilities.
> people often express this criticism by saying "your argument is tautological,"
I have never seen this (nonsensical) way of expressing the criticism that an argument is false by saying that it is tautological (and therefore true).
The GP didn't say that.
As for which informal fallacy the GGP's argument contained, I would say it was more like a false dilemma. And the GGP did say "almost tautological", which implicitly recognizes that there might be other possibilities.
> people often express this criticism by saying "your argument is tautological,"
I have never seen this (nonsensical) way of expressing the criticism that an argument is false by saying that it is tautological (and therefore true).