your guess about what he actually did is as good as mine. gold salts are toxic and corrosive so it would be pretty obvious early on. small gold particles would occlude capillaries so that would be very obviois too. there is colloidal gold and gold nanoparticles are probably not very toxic. that discoloration could be some infection or irritation from whatever injected thing (not medical advice)

My take: The gold probably wasn't the problem. You have to ask yourself, how did he get metal into his veins? It was either dissolved or suspended in a liquid. If it was suspended, that guy was giving himself embolisms by injecting small foreing objects into his bloodstream. If it was dissolved he was probably injecting himself with pure poison.

I mean, let me shove liquid metal into your veins.

I don't think toxicity is the problem.

Paracelsus has famously claimed, the dose makes the poison. His point is basically, everything is potentially a poison. One can possibly get Oxygen or water poisoning even though both substances are vital for our survival.

if its ionic form, or its in salt form it can somewhat poisonous, also gold jewelry is often contaminated or combined with other metals lead, nickel, copper, etc. elemental gold is inert though.

https://en.wikipedia.org/wiki/Metal_toxicity