If you have multiple characters on your account used to store your extra gold bars, check them all.
Check these forum threads:
21 Gb gone from my Inv ?! - Page 4
Ya?anan GB Problemi Hakk?nda
I lost 21GB on a "mule" character.
EditAdd) By the way, OTP is active on my account. None of the GB's on my main character were missing, nor were any items missing in my inventory. This has to be a server side glitch.