As of July 26, 2021, all owners of the Enhanced versions of GTA V / GTA Online will now get the returning player content, regardless of if they're actually returning players.
Starting today, July 26th, it seems that Rockstar has enabled the Returning Player bonuses that were made available to players who previously owned the PS3 or Xbox 360 versions and also purchased the PS4, Xbox One, or PC versions that were released in 2015 to all players, to fix a glitch that removed those free items from eligible players.
There are free vehicles and other items available in Online and Story Mode.
They include but are not limited to:
- Marshall Monster Truck
- Dodo Seaplane
- Kraken Submarine
- Imponte Dukes Car
- Declasse Stallion Car
- Blista Compact Car
- Hatchet
- Duke of Death
Here is a link to the Rockstar Newswire article that details what was made available.
There are also other vehicles that you may find that are now other vehicles on in-game websites that were made free for returning players in 2016/2017 that aren't listed in the original article.
Source: Returning Player Bonuses Available To All Players | Reddit /r/gtaonline
I can confirm that I got the returning player content on the PC Steam version despite not having owned the PS3 nor Xbox 360 versions.
The Native Trainer included with Script Hook V also allows you to spawn the vehicles and weapons exclusive to the 'returning player' content.
Once it is installed, press F4 in-game to show its menu. To make it give you weapons, go to 'Weapon' → 'Get all weapons'. You will then have all available weapons, including the railgun and hatchet in your inventory. To make it spawn a vehicle, go to 'Vehicle' → 'Car Spawner' and then choose the vehicle you want to spawn.
According to its readme, it won't work on GTA Online.
Script Hook closes the game when the player goes Online, this is done because the game reports the installed mods list to R* while being in Online mode.
This is presumably also to avoid getting the player banned if they have the Script Hook installed and are attempting to play GTA Online. It'll only work in the singleplayer story mode and you'd have to disable it to play GTA Online.