@product.GroupPaths.FirstOrDefault().FirstOrDefault().Id
GroupPaths is an array with a nested array containing objects with Id's.
I wanted to get the Id of the first object from this array:
"GroupPaths":[[{"Id":"2500","Name":"Kjeveortopedi"},{"Id":"2580","Name":"Aligner/Tilbeh\u00f8r/IPR","Sorting":0}]]
Result: 2500