supports multiplayer and Quake 2
as well (although the latter one will require you to add an IP address of a remote server in the config file).
In the current firmware, if you want to play locally against a different GCW Zero, both units will need to connect to the same wi-fi network.
The next firmware release (to be released soon) features a unit to unit connectivity without the need of having an active wi-fi network in the area (one of the units hosts it's own network).
I don't know of any RTS games for GCW Zero, yet alone with multiplayer feature.