The MediaTrackSettings dictionary's autoGainControl property is a Boolean value whose value indicates whether or not automatic gain control (AGC) is enabled on an audio track. This lets you determine what value was selected to comply with your specified constraints for this property's value as described in the MediaTrackConstraints.autoGainControl property you provided when calling either getUserMedia() or MediaStreamTrack.applyConstraints().
Automatic gain control is a feature in which a sound source automatically manages changes in the volume of its source media to maintain a steady overall volume level. This feature is typically used on microphones, although it can be provided by other input sources as well.
If needed, you can determine whether or not this constraint is supported by checking the value of MediaTrackSupportedConstraints.autoGainControl as returned by a call to MediaDevices.getSupportedConstraints(). However, typically this is unnecessary since browsers will ignore any constraints they're unfamiliar with.