Gongs are often more for show than they are actually used musically by the people who set them up (assuming we are talking about rock, etc, and not like oriental percussion ensambles or marching band pits).
To truly use a gong correctly, you hit it with a large mallet, but only after softly rolling on it first to "warm it up." Thus, they really aren't meant to be used as an alternative to cymbals. But you can get a great sound out of them. Also, you can get some neat effects when you roll on them. I remember using a gong in high school orchestra, rolling on it, during the Jaws theme song. It represented the calm water that the "dun dun. dun dun. dun dun dun dun dun dun dun dun dundundundun DA DA DA!" part crept over.
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.....
I actually bought my mother in law a 14" wuhan gong for a dinner gong and she uses it all the time.
I use a 28" chau gong on my set, I use it actually in 4 or 5 songs during the set when we are playing my music. I use it with sticks during my solo's. It sonds great at the beginning of one of my songs as I start with a cymbal crescendo 2 measures in and switch to the gong almost at the top of the crecendo and finish with the gong crescendo. They have there place all over a concert if you can tastefully fit it in.
You could use it as an insane opener for a song. Use it with a violin bow across the edge and it makes a wild sound. If I remember correctly, its called bowing(not a very clever name), but I'm not quite sure.