Eating options in Ghomrassen are limited to a handful of small local cafes. This is a small Tunisian town without a tourist infrastructure, and the food reflects that: simple, cheap, and aimed at local workers and passing Tunisian travellers rather than visitors from abroad.
For anything beyond a basic meal — a wider choice of dishes, supermarket supplies, or fresh produce — head to Tataouine or Medenine. If you are staying a while, it is worth bringing supplies from either town.