Well, that didn't take long. Mere hours after launch, Fallout 4, which has had quite an extensive relationship with GTA V, both official and unofficial, has gained more concurrent users than Rockstar's blockbuster.
While most Steam player leaderboards have the omnipresent, typical trio of Dota, Counter Strike and Team Fortress, these games "don't count" in the eyes of the wider gaming community. Instead in the case of AAA games released on Steam, for the longest time GTA V has dominated with the most in-game players at any given moment. Since its release on Steam, GTA V has seen through many high profile releases, and none have ever surpassed it. Even on launch day which obviously sees the largest amount of concurrent traffic.

Fallout 4 launched today worldwide and steadily had well over 200,000 players venturing into the wasteland at the same time, with it almost hitting 300,000 at its peak. In contrast, the number of concurrent GTA V players on Steam sat around the 20,000 mark on average, with the peak still being below 50,000.
Granted, these are just the Steam numbers (and GTA V has a massive console player base), but Fallout 4 is a multiplatform release, and possibly the biggest game to be launched this half of 2015. So chances are that the numbers are similar on Xbox One and Playstation 4.