As far as we know, it's OL only unless you're in NYC, in which case it may be available at one or two stores. It's not clear whether it's online in the US only, or whether it's available in other countries.
The release date is "Black Friday"- the Friday after US Thanksgiving.
The EL gunmetal lippie is sheerer and- I find- a little browner. It reminds me more of Urban Decay's Oil Slick lipstick than this one. Whether you need both really depends more on how often you'd use this type of shade. If you're using it strictly to darken other shades, I'd say you don't need it. If you want to wear the colours, though, I think there's enough difference to warrant having both.