In a very Microsoft kind of move, it appears Windows 10's new Game Mode feature will only work with PC games tied to Windows 10's UWP ecosystem.
Windows Central seems to have confirmed my initial prediction: Windows 10's upcoming Game Mode feature--which allocates system resources on an OS and hardware level and optimizes them specifically for gaming, thereby boosting game performance--will only work with UWP-enabled PC games. This means only PC games purchased on the Windows Store can leverage the benefits of Windows 10 Game Mode, so everyone's entire library of Steam games may not work.
"Previously, we were unable to ascertain whether Game Mode would be a feature restricted to games built for the Universal Windows Platform (UWP), but it does appear that this could be the case. Our information states that games developed for the Windows 10, Xbox One, and Project Scorpio ecosystem specifically need to be "Game Mode enabled" Universal Windows Apps (UWA)," reads Windows Central's report.