That's not possible. Sofia needs you to do a 'sofia profile <name> rescan' to pick up new gateways, and if a gateway has changed its parameters you need to do 'sofia profile <name> killgw <gwname>' then rescan.
There's a reason for it too - FS has to register with the gateway then keep pinging it with OPTIONS requests. That means the gateway needs an object stored in memory. It wouldn't be ideal for FS to constantly poll a database looking for changes for something that might never change, so you're left with rescan.
You could however have a DB trigger that runs a script that connects via ESL and does that for you though...
-Steve