We don't have account merging yet, as while Matrix can see all of Gitter, we're not exposing the entirety of Matrix into Gitter (mainly because it'd cause the Gitter AWS bill to go through the roof :P). Once Gitter becomes a pure Matrix client, though, you'll automatically be able to use your Gitter.im homeserver credentials to access Matrix, and with account portability (MSC2787[1]) there /may/ end up being a way to combine two existing accounts into one.