Arcade mode is offline
In a extended interview with Eurogamer, series creator Kazunori Yamauchi has confirmed the majority of Gran Turismo 7 will be online only and will function similar to Gran Turismo Sport. This means modes such as the main campaign and the new GT Cafe are online only.
Yamauchi states the reason for online connection is to prevent cheaters trying to modify the save data. Anything in the game that requires save data will need a connection. The only exception is the arcade mode, as it does not require save data.
Gran Turismo 7 releases on March 4, 2022 on both PS4 and PS5 and will feature cross-play.