Why another library when input is included in most engines/libraries?

There are lots of other ways to acquire input, most are part of more complete engines or more comprehensive libraries. For one, Gainput is meant for those who are using something without input capabilities (for example, pure rendering engines) or those who are developing something themselves and want to skip input handling.

In the long run, Gainput aims to be better and offer more advanced features than built-in input solutions. That's the reason why more advanced features, like input recording/playback, remote syncing, gestures and external tool support, are already part of the library.