Hi Barnaby!
I'm a GTA (but salaried not paid per hour), and there is absolutely nothing to stop me teaching elsewhere - except a total lack of time! We don't have a contract, or even an informal agreement, but that might be because my uni is pants at admin.
If you're not currently a GTA (but starting soon) then you might find restrictions on your time, if you think it's manageable do it. But it's usually good to check with someone who has helped devise the GTA teaching policy. At ours we have a director of Graduate Studies who is the holy grail of knowledge on this stuff.