I have a few ancestors from that time frame buried in Oak Hill in Gloucester as well - Michael B. Murray and his siblings and children.
I know this is an old thread but have you contacted the Gloucester Historical Commision? They have some info and were likely the ones placing the flags. They made a recent effort to get the original St Anne's Church (the church affiliated with the cemetery) declared historic (which was likely the church your ancestor Patrick Duffy attended, and for sure was were mine was). Sadly St Anne's had a fire at some point and then merged parishes and volunteers had possession of the records for awhile I have had no luck accessing the church records.
The other source you could try is Greely Funeral Services - they are a family business and have been around since the late 1800's - They have FANTASTIC. Records and are very friendly. They may have old cemetery maps, and I got lucky and they actually provided the burial and headstone for my ancestor and could give me a good amount of information on where he was in Oak Hill.
Oak Hill is not laid out in the manner you would expect and I have not found any numbers for rows/plots either.