How to place a gong in my score?

On the New score dialog, under Choose Instruments (top center), under Family (at left), change "Common" to "All Instruments". I found "Tuned Gongs" under Percussion - Pitched. That's probably the best bet. There's also "Bowl Gong" and "Suspended Cymbals" under Percussion - Unpitched. Neither likely to be what you want but the "Suspended Cymbals" wouldn't be a horrible choice (if the "Tuned Gongs" weren't available).

The dialog I described above is the same as Add or remove instruments that you access from the Add button on the Instruments tab (upper left).