It's highly unlikely that GTA 6 will be officially released on Android and iOS devices anytime soon.
While there are some cheeky ways to play a game like GTA 5 on Android and iOS, there are no official ports of said title there. Given the sheer size of GTA 5's file size, it's not surprising that an official port hasn't been made yet.
If GTA 6 is going to be a bigger game than GTA 5, then it's even less likely that it will be officially released on Android and iOS devices.
Given Rockstar's past history with mobile games, it would be a miracle if GTA 6 is officially released on Android and iOS devices.
Why GTA 6 is unlikely to be officially released on Android and iOS
As fun as it might be, GTA 6 is highly unlikely to be playable on Android or iOS devices anytime soon. Rockstar Games hasn't made a serious push to the mobile market in a long time, and they'll likely generate a massive profit selling GTA 6 to their pre-existing markets as is.
Why fans would like GTA 6 on mobile devices
The most obvious reason to enjoy a future game like GTA 6 on mobile devices is that it would be ready to play on the go. Part of the reason why a console like the Nintendo Switch is so successful is because it offers a level of convenience that no other home console can match.
Most people that own modern technology tend to own an Android or iOS phone. Phones are essential to modern life, whereas something like a gaming PC or a home console is fun but not necessary for day-to-day life for most people.
Why GTA 6 won't be on Android or iOS
GTA 6 hasn't even been announced yet. Some leaks are suggesting that it will only be available for the PS5, Xbox Series X/S, and PC.
If that's the case, then a mobile port isn't on the horizon anytime soon.
Once it is released, players will likely figure out a way to play the game on Android and iOS. But it's on the playerbase to figure that out, as Rockstar Games isn't involved in that process.
Past GTA titles on Android and iOS devices
Although the old GTA games have done well for priced games (especially GTA San Andreas), it's worth noting that they're older games that are easier to run. They're also not free-to-play, which is something most mobile gamers tend to avoid like it's the plague.
It would be interesting if GTA 6 had microtransactions and was F2P, as it would attract a lot of players if that was the case.
However, any microtransactions would likely be limited to GTA 6's online mode, which isn't likely to be ported by itself.
It should also go without saying that there is no GTA 6 download on the mobile market. There are a lot of fake links out there for one reason or another, so fans wanting to play GTA 6 should just wait until it's officially announced and released, first.