Hi,as per OH la la,you haven't mentioned what heat you were giving them,they can get wet in there own drinking water,then they will then get cold and die even in a centrally heated house if they don't have a heat lamp or electric hen.
why were you feeding them chopped grass this could well of got impacted in there crop and caused all sorts of problems.
I find on the whole gosling's are very hardy little characters and this is there down full people think they are tougher than than they are.
I hatch dozens of gosling's and I have only ever lost one,it managed to climb into a bucket of water and couldn't get back out,lesson lern't.
Graham.