In a rock setting, 2 spots are effective.
The first note of your opening tune.
The end of your show (preferably setting it on fire, but that's optional depending on your local fire code--and YOUR distance from it
).
Other than that, unless you're doing a long jam like a Zeppelin type thing, where you can do a build, or some stick hits on it for a different sound, it's mainly for looks.
But, if you're doing a rock show, looks and visuals are a part of it, so depending on the type and size, if you don't mind lugging it around, go for it if it'll work for you.
I have a 28" Wuhan Wind Gong, and I'll use it once in a while at our shows, just to mix it up a bit.
I find the Wind Gong nice because it gets the "Gong" sound real fast, and then dies out in a reasonable amount of time, and doesn't go "uuunnnnn" behind you until the song is done.
The 28" gong looks plenty big behind the kit if you want some real visual impact behind your kit, and it's not all that bad to carry around. Meaning, it's not a deal breaker on weather to bring the dang thing or not, and it can just goo off a sturdy cymbal stand with an inexpensive gong holder attachment that runs less than 20 bucks everywhere I've seen it....and, if you can pull of the fire thing.....