That's what I also would like to know!
Nobody answered how top realize an alpha masked env. shader for my car because not all parts or made of metal...
Also the windows should be a little bit reflective + nearly 100% transparent so it's a mix of alpha masked skin and alpha masked env. shader but knowbody answered by now!

Guido Richter Foppsoft Developments Wetterpool.de (German weather site) [work:] { not using 3DGS anymore }