Rockstar North has explicitly stated that Grand Theft Auto VI will not feature microtransactions.
During a conversation with the developers, Andy Cortez from Kinda Funny Games asked Rockstar North head Rob Nelson if players would be able to spend real money to acquire any in-game items. The answer was extremely brief: "No monetization."
Greg Miller also inquired whether generative artificial intelligence was used in the creation of GTA VI. Rockstar's answer to this question was equally unambiguous: "No AI."
As for GTA Online, Rockstar still hasn't revealed any details. The developers declined to answer questions about the next version of the multiplayer, so it's currently unknown whether the absence of microtransactions will persist in the future online component.