What have I missed?

Banteng are established. Gaur and banteng are related. The Cobourg Peninsula is well suited to banteng. Banteng are also established inland from the Cobourg Peninsula. There are huge areas similar in Northern Australia.

There are also banteng behind wire in Texas.

Gaur lived in many areas of India. Deforestation would be the major cause if their decline.

Gaur existed in many other regions, Burma, Thailand, Vietnam, Malaysia. In Malay called Seladang.

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From first hand knowledge I was told in India, one reason for gaurs decline in the parks is they are a favourite prey species for tigers. True? No idea. Allegedly tigers take a considerable toll on them.

I imagine as they are an aggressive bovine species human conflicts might also be a problem.