Since you asked about Gyro, this isn't officially explained in the anime so I don't think it's a spoiler, but this one page of the manga explains what happened to him. Read at your own risk: https://mangabeast.com/manga/Hunter-X-Hunter/0204-009.png
Abara_art
2019-06-30 01:44:42 +0000 UTC
Yeah Rammot learned it because he got hit with it. That's similar to how Wing taught gon and killua by injecting them with aura.
Abara_art
2019-06-30 01:31:27 +0000 UTC
Killua said he was immune to poison during the Hunter exam after the rookie crushee gave him that drink filled with laxitives