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