To access Sync Gateway while running in an emulator, you need to use the special address the emulator recognizes to connect to localhost (see https://stackoverflow.com/questions/5528850/how-to-connect-localhost-in-android-emulator?lq=1 for the standard Android emulator).
What versions of Couchbase Lite/Sync Gateway/Couchbase Server are you running?
And of course you need to have a replication active in your Android app.