The Chinese population was concentrated in Xishuangbanna, near the Laos border. Survey work through the 2000s recorded steadily fewer calling groups, and the last widely accepted evidence dates from around 2007 to 2008. Gibbons are unusually easy to census by sound, since groups sing loudly at dawn from fixed territories, so an absence of calls across repeated survey seasons carries real weight.
The cause was habitat conversion combined with hunting. Rubber and tea plantations replaced lowland and hill forest, and remaining fragments were too small and too separated to hold viable groups.
One caution belongs with the claim. Declaring a local extinction is a statement about survey effort as much as about animals, and border populations in northern Laos are close enough that dispersal into China is not impossible. The practical position is that no resident Chinese group has been confirmed for well over a decade.