Song/display call (male): a low, rasping "nhech" — discreet and often overlooked during the display period.
Contact call (female): the characteristic "gack-gack-gack" — like the female Mallard's quack but drier, faster and with a distinct consonant onset; the most reliable identification call for the species.
Alarm: a shorter, sharper "kack" when disturbed.
Listen to the Gadwall's calls on xeno-canto and see observations on Artportalen (the Swedish species observation system).