Actually Rabban Gamliel lived in 30, sorry.
Which is way before not way after.
And if you would bother looking at the Gemorah you would see that it’s pretty hard to state that the calculations were known since B”D was going to accept the Eidim and declare RC and RG did not allow based on his Mesorah.
The Gemorah further indicates that RG’s ruling needed public enforcement by Rabban Gamliel giving a Hesped on what would have been Rosh Chodesh, which pretty much indicates the calculations were not a “know entity”.
You really really should try learning things inside first and not rely that in turn rely on excerpts.