I think it isn't impossible and it would be a megaton to have it in the Showcase, but I think Take 2 will announce it any random day they choose instead, not in a platform holder event.
On top of this, I think Sony obviously would kill to have GTA6 in their marketing wagon with stuff like having a new GTA6 tariler as a highlight of a random State of Play.
But for this particular showcase Sony will try to keep all the attention focused on their 1st/2nd party games and 3rd party exclusives.
Jimbo was talking there about the GTAV PS5 version deal they had, not about GTA6.