St Peter's Free Church, Perth Road Dundee has a burial ground. I don't know if this has been taken over by Dundee City Council for maintenance (it is probably long closed to new burials) but the church itself has a web-site.
I suggest you try www.stpeters-dundee.org.uk
and use the "contact us" facility to find out if there is a monument to John Gentles. They might even provide a transcription if any exists.