Don't get your hopes up... for all we know, this might just be Beta content that was cut from the final game. Until Rockstar announces something official, all we can do is speculate.
Even if it isn't from the Beta, it pretty much sounds like it's GTA Online, and not singleplayer. Which probably means that it will be paid DLC. (Just because we got Beach Bum and we're getting the Content Creator for free doesn't mean that the rest of the DLC will be free, too.)
I'm posting this because there seem to be many members that are not aware of this checklist.
Just go to http://socialclub.rockstargames.com, click on your profile, then scroll a bit down and click "View Stats" on the right side of the page. After that, simply click "Checklist" and there it is, the checklist for completing the game 100%.
Just beware that you need to have a Social Club account and you also have to link your game account to it (your PSN or XBOX Live gamertag).