Currently Online 
From Chartz
Twitch CEO Dan Clancy in an interview with Bloomberg said the company has been talking with Rockstar about their plans and said there will be a big bump when the game launches, with a second bump next year when the multiplayer releases.
"We spent a lot of time talking with Rockstar about their plans, and also helping them think about how they can engage our creators," said Clancy. "You are going to see this big bump with GTA 6, but then an even bigger one next year when they launch multiplayer."
I am assuming that like GTA 5 the multiplayer mode will be apart of the main game and not sold as a seperate purchase. There is always the possibility that the online mode is aslo sold seperately (for like 20USD insteal of 80USD) or is included in GTA+. But assuming it is part of the original purchase, this will generate another huge spike again in 2027.
For folks predicting units of GTA 6 by the end of 2027.
