Hi Krusty
We don't have medicinal bath available in Oz as far as I know.
Maybe to help Gringo(it sounds like scabbing rather than infection), you could have a bowl of double-strength salt water available for him to soak in as well (and his fresh water too)?
Have seen something similar a fair while ago with a crabs leg being a dark colour. A few weeks later the crab dropped the leg and the inside of it was black, that explained why the exo on the leg was a odd shade of grey. Perhaps Gringo's leg has already died and it's just a matter of time before he drops it.
Gringo should be fine, Aussie crabs are pretty tough and you're doing a great job.