There might be terminology differences here.
The Imfield GG-1 body is not diecast. Diecast is pot metal, or Zamack, which is forced under high pressure into a precision two part (inside and outside) tool steel mold at liquid temperature. Just like steam locomotives. Very expensive to make for a garage operation.
The body of this Imfield S gauge GG-1 is made from epoxy resin, cast in a simpler type of single part mold, possibly plaster or RTV, like a slush mold. Much easier for a cottage/garage industry to make.