The upper concourse at Gillette Stadium includes three men's and women's restrooms on each side of the field. The restrooms are located roughly once every six sections.
If you're seated in Section 309, Row 16, you'll walk down the stairs to another small set of stairs that leads to the concourse. From there, the women's restroom is just a few feet to the left and the men's a few feet to the right. Altogether, you'll walk down about 25-30 steps and then another 40-50 feet to reach the nearest facility.