Here are some brands of metallic tint which claim to NOT intefere with GPS:

http://www.tinttrimfactory.com/automotive/3m-tint.htm

and:

http://www.campbelltint.com/automotive.htm

I guess it just depends on the type of metallic tint. I suppose if your GPS antenna is on the front dash or goes through the untinted front windshield it would be OK too. Just be knowledgable.....