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