Hi,
Welcome to the forum and I am glad to help you out. Peppermint instead of making the gallstones worse actually is known to help in the dissolution of the gallstones.
It also is known to have some spasmolytic action which facilitates the resolution of the spasm/colicky pain of the stones. Hence, there shouldn't be any problem in continuing to drink the peppermint tea. I sincerely hope that helps. Take care.