quick_grid_grouped_join.rb

grouped_join = -> join:, group_join:, group_count: 3 {
  -> list {
    list.each_slice(group_count).map { _1.join(join) }.join(group_join)
  }
}

# 9x9 Game Board Grid
grid = [[*0..8]] * 9

cell_seperator      = ' '
col_seperator       = ' | '
segment             = '-----'
segment_seperator   = '-+-'
row_group_seperator = "\n"
row_seperator       = "\n#{([segment] * 3).join(segment_seperator)}\n"

grid
  .map(&grouped_join[join: cell_seperator, group_join: col_seperator])
  .then(&grouped_join[join: row_group_seperator, group_join: row_seperator])

# STDOUT: 
# 0 1 2 | 3 4 5 | 6 7 8
# 0 1 2 | 3 4 5 | 6 7 8
# 0 1 2 | 3 4 5 | 6 7 8
# ------+-------+------
# 0 1 2 | 3 4 5 | 6 7 8
# 0 1 2 | 3 4 5 | 6 7 8
# 0 1 2 | 3 4 5 | 6 7 8
# ------+-------+------
# 0 1 2 | 3 4 5 | 6 7 8
# 0 1 2 | 3 4 5 | 6 7 8
# 0 1 2 | 3 4 5 | 6 7 8