My son loves his GSG. Do they make a 1/2x28 to -14mm thread adapter? Basically I'd like to mount an airsoft fake suppressor. Let me know and a link to buy. Can't find any info on one.

You're not likely to find any adapter that will readily allow you to attach a 'fake' airsoft suppressor to your real weapon, as you're skirting with a Federal firearms violation if you do. The BATF had ruled against all foreign manufactured airsoft fake silencers as early as 1994. If they come apart readily or can be modified easily they are 100% ILLEGAL. Any fake silencer (for airsoft, toy or real weapons) having any void / cavity / open space on the inside where gas can accumulate it is 100% ILLEGAL here in the USA.  Any airsoft fake silencer that has removable end caps is also 100% illegal to possess in the USA. Just possession of either an airsoft fake silencer tube or end cap (no matter the material of construction) is a felony which can land you in a Federal Penitentiary for up to 10 years.
BATF does not fool around with this issue and the new Homeland Security / Patriot Act gives greater latitude to Federal Prosecutors, as it is considered felony possession of an unregistered Title II device. As this is a very serious weapons violation (same penalty as an unregistered live machine gun) any Law Enforcement Agency would be happy to catch those who break the law in this area, mostly because it is a slam dunk in court for a Felony Weapons Charge.
Personally, I wouldn't take the chance of spending up to ten years in prison just because I wanted my gun to look cool. Bite the bullet and purchase a real suppressor legally. You won't be sorry.

What he said, plus many of the airsoft fake silencers are made of pot metal or other inferior quality metals, and I would not trust them on a real weapon.