I've sent this e-mail to Rockstar:
I'm a loyal PC player and have never owned any console, and thus I'll not be eligible for exclusive content only for returning players (railgun, Kraken sub etc.). I don't like it that I'll never be able to access this content just because I don't play on consoles, so I'm asking you - are you planning to release this content as a paid DLC in the future for those who didn't play the old gen versions?
Thanks for your email. The exclusive content is only available to players who have played GTAV on last generation consoles and current generation consoles.
Cheers, Rockstar Games
It seems exclusive content will also be available for people from Xbone and PS4, not just Xbox 360 and PS3 like they announced.