Considering the game 'GT Sport' has been announced, and is on the GT website product page, I find it highly unlikely this game won't be available in stores.
If it's available in stores, it's able to be played offline.
(Almost without exception)
This 'online only' thing, IMO, needs to stop.
I've seen one post about it, and it has no backing what-so-ever.
There's no reason to believe this is the case.
No PS4 game, to my knowledge, is onine only.
(And even if proven wrong, would be an isolated case.)
I see no evidence why this game would break that mold.
And can think of many reasons why it wouldn't.