The first finding of fossilised plant gum has been found in the Crato Formation of Brazil. Belonging to the extinct plant Welwitschiophyllum brasiliense it is dated from around 110 mya.
Here is an article about the finding
http://www.sci-news.com/paleontology/fossilized-plant-gum-08163.html
The scientific paper is listed below and is open access
https://www.nature.com/articles/s41598-020-60211-2