A gong is cast from a bronze alloy and then, unlike a bell, tuned afterward by hand hammering, a slow cycle of heating, hammering and re-checking the pitch that a skilled gong smith carries out by ear rather than by measurement alone.
Where the craft is concentrated
Javanese gong making, tied closely to gamelan ensemble tradition, is historically a specialized and in places hereditary trade, distinct in its methods from the foundries that cast temple and church bells even though the base metal is similar, and Chinese gong making follows its own separate line of craft and tuning technique. The two traditions are not interchangeable skill sets, a bell caster does not necessarily know how to tune a gong, and the reverse is equally true.
Who plays them, and their long working life
In temple use a gong is struck by attendants or monks at fixed points in a service, while gamelan gongs are played by trained ensemble musicians in a setting that blends devotional and performance functions rather than separating them cleanly. A large gong is typically a valuable instrument passed down for generations, and a cracked one is far more often sent back to a gong smith for re-tuning or repair than discarded and replaced, since the tuning represents real accumulated skill that a new casting would have to earn again from nothing.