Hi,
Welcome
I expect someone on here can give you some idea of who to contact in Manchester about the grave.
I expect you have found the plan which shows you where GG R/C (attached)is situated?
Possibly a long way round but I wonder if the graves are in numerical order you might be able to narrow the area down ( I think it is a big area!)
Have found two plots GG Roman Catholic 2003 and 2033 names Leavy and Waldron.
Photo of plot 2003 gravestone perhaps you could search for that, it is fairly distinctive.
Plot 2033 has a photo but it is not a gravestone, just a small metal flower holder.
www.findagrave.comLike I say there might be a much easier way!
If you need some help accessing the photos let me know.
Cathy