William Marshall's gravestone is in St Cuthbert's Graveyard and marks the grave of the 120 year old 'Gypsy King' who died in 1792. Coins left on top of the gravestone may be part of gypsy custom which has endured at the grave over the years.
For more information and tour details St Cuthberts Graveyard check out Kirkcudbright History Societies website:
http://www.kirkcudbrighthistorysociety.org.uk/news/kirkcudbright-kirkyard-tours/
Theres also an entry on the Canmore website for the graveyard:
https://canmore.org.uk/site/64069/kirkcudbright-st-cuthberts-church-and-graveyard