Судя по этому ответу, не активированные ранее в GfWL ключи дисковой версии можно использовать в Rockstar Games Launcher уже сейчас. Активированные — где-то с апреля (точная дата не указана).
Physical Media:
Games not previously activated using Games for Windows Live will be able to use the Activation Code on the back of the game manual to activate and update to Grand Theft Auto IV: Complete Edition
- Download and install the Rockstar Games Launcher: https://socialclub.r...-games-launcher
- Sign in with your Rockstar Games Social Club account
- Click your avatar in the top right
- Select Redeem Code
- Enter your Rockstar Activation Code into the field
Games previously activated using Games for Windows Live will require players to create and/or link their Social Club accounts in replacement of Games for Windows Live to update to Grand Theft Auto IV: Complete Edition.
- Download and install the Rockstar Games Launcher: https://socialclub.r...-games-launcher
- Sign in with your Rockstar Games Social Club account
- Link Social Club and Steam accounts
- Click your avatar in the top right
- Select Redeem Code
- Enter your Rockstar Activation Code into the field