So, Wikipedia is not a bad resource in this case. They cited a Nature article that provides good evidence that the story about the monks was partially true. Monks in Tianmu Mountain were transplanting Ginkgo Biloba when they were very scarce, but these trees are not the sole progenitors of today’s Ginkgo.
“It has been suggested that West Tianmu Mountain was the refuge of G. biloba during the last glaciations, although there has been some disagreement (Liang and Li, 2001; Lin and Zhang, 2004) due to lack of a detailed fossil record (Zhou, 2003).”
“The most likely refugium is sited in Jinfoshan, at the boundary of Chonqing Municiplity and Guizhou Province, as suggested by the current occurrence of ancient Ginkgo trees. Recently, a natural-like Ginkgo forest was reported in Jinfoshan by Li et al (1999)”
“West Tianmu Mountain is famous for temples. The oldest Old Temple, was built in 936 AD The largest ginkgo tree is 123 cm in DBH and estimated to be about 500 years old. Hence, the Old Temple was built much earlier than the largest ginkgo tree. Of the many tree species planted around temples, the Buddhist monks preferred Ginkgo. As a result, there are many large ginkgo trees in or around temples in China, with some more than 1000 years old. Therefore, ginkgos in West Tianmu Mountain are likely to have been planted by monks, and may not be natural.”
Based on this, it is not true that there was one Ginkgo tree being cared for my monks in China that saved the species. This myth probably grew out of the discovery of Ginkgo by Western botanists.