When I play Gust and it gets countered, does it still count as a card played for Darius Trifarian effect to ready him?

No, playing Gust and having it countered does not count as a card played, and therefore it will not trigger Darius, Trifarian's ability.

According to Rule 425.1.b:

"A card that is Countered is not considered to have been played."

Additionally, RiftJudge FAQ #9943 clarifies:

"If a game effect prevents the resolution of the card—for example, because the card was countered—the card wasn't played and no abilities that trigger on playing cards trigger."

Sequence of Events

1. You play your first card of the turn.
2. You attempt to play Gust as your second card of the turn.
3. Your opponent plays a card with a "Counter" effect (such as Defy) in response.
4. The counter resolves, negating Gust and clearing it from the chain.
5. Because Gust was countered, it is not considered "played."
6. Darius, Trifarian never sees a second card being played, so his trigger condition is not met, and he will not ready himself.