There is a memorial at Nebo School. It has 'William R Roberts, Pant Gwyn 15 9 1915' I'm having a struggle to positively identify him to that date at the moment.
Is he your great uncle? If not, what is his name/date died.
Incidentally, in case it's another branch, there is a Thomas P Griffiths, (father John P Griffiths,Noel St, Lodge Lane, Liverpool) who died of dysentery with 11th Bn RWF on 24 6 1917. His mother is shown as Ellen Griffiths, Bronhaul, Llanllyfni on CWGC. This could be a different Bronhaul of course
Gwil