opposingforces: you don't have to. claim it via the website. when you claim the code on prime gaming, it gives you the code and a link to the rewards page for cyberpunk. enter code and the gun will be added to all your stashes.
I did all that:
Amazon Prime -> get key -> cyberpunk.net -> redeem -> success
...but how can the cyberpunk.net site add anything to my ingame inventory without some sort of connection or intermediary (such as GoG Galaxy or a separate download)?
If the redeem site really did that directly to my otherwise offline game saves, I would be seriously concerned regarding their online security scheme.
EDIT:
My fault, I didn't read the fine print: Phantom Liberty is required. I don't have have the expansion yet (getting a torrent for it now), only the 2.0 patch.
Still, I am having concerns regarding how the item is to be added to my local game install, once I get ahold of Phantom Liberty.
If all else fails, I'll just wait for the updated CET console codes and add the weapon that way myself.