Last month, MSPU reported that Microsoft is planning to bring Game Mode in Windows 10 with Creators Update. Now they have posted screenshot on how to enable the option. Though Microsoft officially did not unveil it but if you install Windows 10 Build 15007 then you can use this new feature.
According to MSPU, on hitting Win+G you will see Game DVR Panel pops up. Pressing on the settings icon on the bar will led you to new Game Mode Settings. While the feature does not work yet but a BEAM icon ( a game streaming service from Microsoft ) is also present on the bar which is supposed to let you stream games easily though its disabled now.