Question about GTA 5 - 103

This answer is wrong! The key for the boxed version can only be activated on Rockstar, not on Steam. The game itself is no different, and Rockstar players can play with Steam players in online mode. However, the box version does not support any Steam integration, it all runs via the Rockstar Games Social Club. That was the same with Max Payne 3.

EDIT:
The key can only be activated via the Rockstar Social Club.

You can also activate and download the game on Steam by entering the included key on Steam without using the discs. However, depending on your internet connection, we would recommend installing through the discs.