What's this game featuring a 15x15 peg board, and 60 pegs for the two sides?
Use of pegs instead of "stones" suggests Gramps was imitating the commercial game "Pegity", which predates Pente by a number of years.  Pegity was played on a hole grid, with pegs (all plastic when my family played it in the late 1960s and early 1970s, though apparently originally wood).

I haven't seen a set in forty years so don't recall the grid dimension.  The main play difference between Pegity and go-moku (and the later, unexpanded Pente) was four different colors of pegs, at least in the plastic version -- the game still revolved around forming a row of 5 on rows, columns, or diagonals.