Was the word "guy" gender-neutral in the 1930s?

The Corpus of Historical American English lists only 45 hits for swell guy, all referring to men. This, of course, is too small a sample to infer any particular usage with certainty, but it seems to me that the character in question could just as easily have referred to herself ironically as a boy scout: i.e., exhibiting the opposite of various qualities listed in the Boy Scout Oath. Swell guy is then a briefly ungendered metaphor for a code of upright, trustworthy behavior — though swell guys were not averse to fun.

Given Hammett's hypermasculine style — indeed of the whole genre of hard-boiled detective novels — it's not surprising that a female character would refer to herself with a term usually reserved for men.

When women of that period were fun, dependable, and trustworthy like swell guys, they were likely more often called a good egg, a term used for both genders.

Singular guy is still gendered for most Americans, who would only refer to, say, an injured animal as a "poor guy" if the animal is male or of unknown gender. Guys in the plural may be perceived by those who use it as ungendered, but not in all circumstances. You guys as filling the same second person plural linguistic gap as Southern American y'all is more readily seen as an inclusive use.

On balance, I'd say the chances of Hammett's using swell guy inclusively are slim to none.