Indeed, the Greek borrowing was mostly used an a neuter noun in Latin but occasionally we see examples of chaos used as a masculine noun (one source claims that there is only one use of chaos in Latin as a masculine noun - Vetus Latina Luca 16.26 but I couldn't verify this). We need to look it up in TLL.
Quite often, we can't easily tell whether it was used as a masculine or neuter noun though. For example, Ovid writes in Metamorphoses II:299:
si freta, si terrae pereunt, si regia caeli,
in chaos antiquum confundimur! eripe flammis,
si quid adhuc superest, et rerum consule summae!”
As we know, the Greek word Χάος is neuter. Here's its entry from Montanari:
Caduff 2006 describes it as "which is no further reducible but nevertheless exists" (Brill New's Pauly).
Even though its etymology is not entirely clear, the communis opinio is that it is probably related to the verb χάσκω 'be open, gape wide.'
Thus, Beekes (2009/2016) speculates that "an original meaning 'hole, empty space, yawning' is quite thinkable for Χάος."
Because of its connection to the verb χάσκω, some researchers speculated that Chaos can be seen as an essentially feminine symbol. But imho this is highly speculative and there is no clear evidence to verify this claim.