could I remove all hidden games at once?

You can turn off hidden in settings > giveaways > Hide Games you manually filtered

You will need a third party tool like Depressurized.

EDIT Are you asking about Steam or Steamgifts?

NECRO!
use this on every page (copy paste in console)

var buttons = document.getElementsByClassName('table__remove-default');
for(var i = 0; i <= buttons.length;i++)
    buttons[i].click();