User has to define windows processes connected with Gaming Mode. Application monitors for these processes and if found user-defined brightness and contrast values are applied.
There is also possibility to set temporary brightness and contrast from the taskbar:

Alter

You don't have to use your monitor's OSD anymore!
Temporary settings are overridden on next profile change.

Known issues!

* Display driver often reports too wide possible contrast regulation range to the application. For example my Acer Predator X34 reports available range from 0 to 150 while only values from 50 to 150 take effect (actual 0 to 100 display's range). Setting anything lower than 50, in my case, results in actual contrast value 0 on the display.
* On multi-display configurations only the primary monitor is supported.