I don't see where OSAS came from Gnosticism.
It didnt.
OSAS, came from Religious Jews in Paul's day....who wanted to confused Christians about the Gospel.......So, they put a spin on it.. and this spin captivates the carnal flesh in the type of Chrisitan who is trying to stay saved by self effort, or law keeping or commandment keeping.
So, that type, wont have God's Grace, as they are SELF Righteous, and want to prove that THEY are in control and its up to THEM if they stay saved.
So, these Jews, in Paul's Day......understood this Self Righteousness in many Christians...., so, they created this "subverted Gospel" concept for THEM that teaches ....>"you are teaching that you can do anything you want, and not have any consequence""
See that? = OSAS.
So, the person who is involved in trying to stay saved.....hears that false gospel and they LOVE IT........>"oh YES" !!!.....becasue that teaching denies that God's Grace that is Jesus on the Cross is the GIFT of Salvation that both saves you and keeps you saved.
See these "OSAS obsessed" who hate the idea that God's Salvation is permanent and they can't earn it or lose it..... will not have "The Gift of Salvation" as that does not include their self effort and self righteousness.
So, its the LEGALIST who is the "OSAS"... fanatic, who always has to try to twist God's Grace into that early Jewish subversion false teaching of God's Grace.