Why do you need BWI? There is no public transportation between Baltimore and Washington Metro. You will have to pay extra for using private taxi/shuttler.
You need to go to DCA then go to Metro rail right next to National (Regean) Washington Airport then hop on the Yellow line subway then change at Chinatown/Gallery Place Station and then hop on the Red line subway to New York Ave Station for several stops. It is only 5 blocks walk to Gally on Florida Ave.
If you wish to go to a great shopping mall, I strongly recommand you to go to Pentagon City Metro. You are gonna love it...
Have fun.
