On Tzam Gedalyah there is a bris and it is on the fast day at 4 can all the participants eat at the bris (150 people).
Answer:
The participants may not eat.
There is a dispute among poskim concerning whether or not the ba’alei bris can eat on Tzom Gedaliah (the Mateh Efraim 602:3 is lenient, but the Mishnah Berurah, Biur Halachah 549 is stringent), but concerning other participants all agree that they cannot eat.
Best wishes and mazal tov!
