Samsung's Game Recorder+ app lets you record games on the S6 and other Galaxy handsets21
Game Recorder+ has a "floating UI" that makes recording easy, and allows users to activate the front-facing camera in order to add their commentaries in real time. Game audio can be recorded "directly from the source", thus without any background noise, though this is supported only on the S6 and S6 edge. After they're recorded, videos can be trimmed and uploaded on YouTube. The new application even has a function called Game Boost, which reportedly "maximizes memory usage."
According to Samsung, Game Recorder+ is available in 161 countries via Google Play and Samsung Apps, and can be used on the following devices (besides the S6 and S6 edge): Galaxy Note 4, Galaxy S5, Galaxy Note 3, Galaxy S4, and Galaxy Note II. Have you tried Game Recorder+, or do you plan to?
sources: Samsung Tomorrow, Google Play