Here is the full list of all 15 Gym Simulator 24 achievements.
Win a lifting competition against the open air gym bros
Open and fill the pool
Break the Second Gym's mid wall
Buy the Second Gym
Buy the Boxing Club
Play gambling and win
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
Place first decoration item
Buy first gym equipments
First Opening