Moss,
I cant find the photos from Gray Hill at the moment, hopefully they will turn up.
However here is some information that may assist you.
1. There is no geological that interupts the line of site from Gray Hill to Kelston round Hill.
2. Gray Hill = 550ft
Kelston Round Hill = 683ft
3. Places of interest it passes through;
Bang through the middle of Caerwent (roman town, fort) Got some excellent photos of Caerwent BTW. I will get cussed at it posted them here :-(
Fort at Sudbrook
A Long Barrow at Lockleaze (but this is not in the line of site)
4. 24 Miles apart (topo distance)
Hope this helps,
Ike