First of all, the comma is in most cases only a guide to where a speaker would pause during conversation. As you know, there are several punctuation marks that serve this purpose -- the comma, the colon, the ellipsis, the semicolon, the dash, and others -- and each has a different tone. The comma is a brief pause, the semicolon a longer pause, and the ellipsis a still longer pause. The tone of the colon and the dash vary slightly with context, although the dash is usually dramatic while the colon is didactic.
There is no defined rule when you should use one or the other. In the end, you choose based on your own personal writing style. Personally, I rarely use the semicolon, and have started to use the dash less often. I probably overuse the colon, but then I tend to lecture when I write.
My point is that you can't only use punctuation as your guide to the meaning of a sentence. You have to consider it as a whole.
In your first sentence, yes, "even gullible" is extra information added to the end of the sentence, after a short pause to indicate a rephrase. But the second sentence is different. "Life" refers back to the earlier existential "it", and is essential information. The sentence could have been phrased:
Life is what she feared losing the most.
but delaying this key detail to the end of the sentence is more dramatic. Another example:
It is what we want most in this world, pizza!
I think a dash here sounds better than a comma:
It is what we want most in this world -- pizza!
but this doesn't really change the meaning of the sentence, only its tone. Someone with a different writing style might prefer the comma or write the sentence in a way that they didn't feel obligated to use a dash.