There isn’t some specific paragraph about the “being in possession of illegal gold”.
The cincher is that while there are things mentioned in the EULA, COC and TOS, none of them are meant to be exhaustive. They touch on the big issues, because there are so many nuances that can come into play at a later time - they can’t come up with every single scenario that is going to put people’s accounts in jeopardy. People are creative when it comes to finding ways to circumvent the rules.
As Orlyia pointed out, there are ways to protect ones’ self if they are mailed illegal gold. Another player ran into this issue earlier this week. They reported the incident, that their add-on removed the gold from the mailbox and it was not something they solicited or anything of the sort and wanted it to be removed. It’s not a guarantee that a sanction won’t happen, but it creates a “paper trail” and shows that you’re being proactive in dealing with it, rather than just keeping the gold or spending it before you can get dinged on it.