The open access journal Chinese Science Bulletin has an article on the carbon isotope values of Gigantopithecus blacki teeth.
The study looked at samples from four G. blacki teeth (from Longgudong Cave – an early Pleistocene site in China) along with 24 teeth from associated mammal fauna. Long story short G. blacki was primarily a C3 consumer and indicated that G. blacki lived in a dense forest environment. Here is a chart comparing carbon isotope values of various hominins:
Stable carbon isotope analyses indicate that G. blacki from South China had a diet of pure C3 biomass resources, and lived in dense forest habitats. The diet and habitat of G. blacki was significantly different from that of early hominins in Africa, such as Australopithecus and Paranthropus, which could live in open habitats and consume both C3 and C4 resources. Dependence on forest habitat might be an important factor that drove Gigantopithecus extinction when the climate and environment changed dramatically during the Pleistocene.
The authors argue that climate changed caused defragmentation of the forests causing the extinction of G. blacki.
For Further Reading:
Ciochon et al (1990) Opal phytoliths found on the teeth of the extinct ape
Gigantopithecus blacki: Implications for paleodietary studies. Proc. Natl. Acad. Sci. USA Vol., 87, pp. 8120-8124
Frayer (1973) Gigantopithecus and Its Relationship to Australopithecus. AJPA 32: 413-426
Koenigswald (1952) Gigantopithecus blacki von Koenigswald, a giant fossil hominoid from the Pleistocene of southern China. Anthropological papers of the AMNH ; v. 43, pt. 4, 295-339
Weidenreich (1945) Giant early man from Java and south China. Anthropological papers of the AMNH ; v. 40, pt. 1, 1-134
Wen-Chung (1957) Discovery of Gigantopithecus mandibles and other material in Liu-Cheng of Central Kwangsi in South China Vertebrata Palasiatica, Vol 1, No 2
Zhao et al (2011) Enamel carbon isotope evidence of diet and habitat of Gigantopithecus blacki and associated mammalian megafauna in the Early Pleistocene of South China Chinese Science Bulletin, November 2011 Vol.56 No.33: 3590-3595