For what it's worth, the history of (this sense of) the word guy is short and it starts very clearly with a man: Guy Fawkes. So to start with, the word "guy" as we use it today to refer to a person, most likely a man, is derived from a man's proper first name.
The OED definitions are, in order of first-attested date:
1806 - An effigy of Guy Fawkes traditionally burnt on the evening of November the Fifth, usually with a display of fireworks.
1836 - A person of grotesque appearance, esp. with reference to dress; a ‘fright’.
1847 - A man, fellow. orig. U.S.
My conclusion based on the attestations provided in the OED as well as perusal of newspaper corpora is that the word was probably more firmly a reference to men in the 1930s than it is now.
Furthermore, it appears to me from the quote in question that the word "guy" is actually meant to mean, quite literally, a man as distinct from a woman.
She rubbed her face into the chintz cushion under it and said: “A swell guy I turned out to be, promising to marry him yesterday and then leaving him to take the first tramp I run into home with me.”
Here the speaker is ridiculing both men and herself by comparing her behavior to men and describing it as dishonest and unfaithful. Her use of the word tramp refers not to "a person on the tramp" but rather the U.S. slang meaning:
A sexually promiscuous woman.
These clues all suggest that the word guy here is used to mean a man, in a fashion consistent with the word's historical trend.