I have had a look at the marriage index for Ffestiniog and the indices for the surrounding Merioneth parishes, but could see no sign of a Griffith Williams marriage in any of them circa 1811.
The difficulty with searching for ancestors from Blaenau Ffestiniog is that it grew up from being a place of just a few farms to a town of over 11,000 souls in a very short period so the vast majority of people who lived and worked there had moved in from other areas. (In 1801 the population of the whole parish was just 732, most of whom lived in the village of Llan Ffestiniog - in 1881 the population was 11,274 most of whom were in Blaenau).
Is there anything in your knowledge of Griffith and Elinor that might suggest where they were born. Were they still around at the time of the 1851 Census? If so this might give a birthplace. Even 1841 should tell us if they were born in the county of Merioneth or not which will help decide the value of going through the marriage indices for the whole of the county.
