Here is the full list of all 15 Gym Simulator 24 achievements.
First Opening
Complete all of start tutorials
Clear all trashes and dirts
Slap a pervert customer
Buy first gym equipments
Place first decoration item
Buy and open vitamin bar
Fill the protein machine
Release rats while breaking the vitamin bar woods
Play gambling and win
Win a lifting competition against the open air gym bros
Buy the Boxing Club
Buy the Second Gym
Break the Second Gym's mid wall
Open and fill the pool