Quick Google search turns up this

"Griffith Evans, High Sheriff of Merionethshire in 1788, was a member of the prominent Evans and Oakeley families of Tan-y-bwlch. While a full family tree is not readily available, it's known that he was associated with the Oakeley family through marriage and likely had connections to other notable families in the area like the Lloyds and Wynns"
There is also a link to an article about EVANS, GRIFFITH, and OAKELEY, families, Tan-y-bwlch, Maentwrog, Meironnydd.
Not sure if it helps but does possible give you other avenues to follow via other family connections. Good luck and happy hunting
