There are several broad categories of gong, although there's plenty of crossover between them.
Symphonic Gongs
Symphonic gongs are perhaps the most recognisable type in Western orchestral and musical settings. They're generally large, complex-sounding instruments capable of producing powerful swells, deep fundamental tones and a huge amount of sustain.
They're available in a wide range of sizes, with larger models generally producing a deeper and more expansive sound. A 32" symphonic gong is often considered a particularly useful starting point, offering a good balance between sound, portability and price.
Planetary Gongs
Planetary gongs are tuned around specific frequencies associated with the planets, moon or other celestial bodies. They're particularly popular for meditation, sound therapy and wellness applications, but can also be used as musical instruments.
Both Paiste and Meinl offer extensive planetary-tuned ranges, with each gong providing its own tonal character and frequency.
Wind Gongs
Wind gongs, sometimes known as Feng gongs, have a flatter profile than traditional Chau gongs and generally feature little or no raised rim.
They can produce bright, explosive attacks alongside long, shimmering decays, making them particularly effective for atmospheric playing and sound therapy.
chau Gongs
Chau gongs feature a distinctive raised rim around their edge and are traditionally associated with Chinese gong manufacture.
They can produce a powerful initial attack followed by a broad, resonant decay, making them useful in both musical and ceremonial settings.
Sound Creation & Speciality Gongs
There are also plenty of gongs designed specifically to create unusual tonal experiences. These can include chakra gongs, decorative gongs, sound-creation models and other specially tuned instruments.
These are particularly popular for sound therapy and meditation, but there's nothing stopping you from incorporating them into a musical setup if you like the sound they produce.