The closest Metro stop is the DuPont Circle station and depending on where you exit the station, it could be a 2-5 block walk.
A quick note: The downside to finding a good place to tour on your lunch break that depends on the Metro is that the Metro has crappy off peak service. Sometimes you have to wait 20 minutes for a train and if that's the case, you end up losing a lot of valuable time. Just something to keep in mind.
Luckily the Metro moved pretty quick and my walking pace can border on running for some and I got to the lobby of the gallery in no time. The permanent display is free but if you want to see the special exhibit it's $12. Also, they strongly encourage a donation of any amount and only after a few back-and-forths of 'I don't have any cash on me' will they let you in for free.
There's a lot to the gallery that I missed because I was practically running through it (worried about getting back to the office on time) and would certainly like to go back and [try to] truly appreciate the art. If I had to guess, I probably saw about 73% in total.