Gisu's side quest is started by talking to her out in the Quarry. She will ask you to find three psychoseismometers, one in the Quarry and two in the Questionable Area. Each of these are combat challenges, where you must fight a couple waves of enemies in an arena. After completing all three, return to Gisu to complete the quest.
Timestamps:
0:00 - Talk to Gisu
2:03 - Psychoseismometer O99
4:23 - Psychoseismometer Q33
5:42 - Psychoseismometer H18