currently not in service, but there are starting after corona break some trains in this area too, like bulgaria-turkey. 

You can use busses on the sofia-thessaloniki route (not included in the Interrail). 

Or is youre really train-mad and want to get the best out of it-bridge the around 20-30 Km gap between end of GR trains and border by taxi or local bus (info about these unclear). IUt will lengthen travel time a great lot and probably even hardly save money