"Gym Rat" "Workout Freak" those are just some slang terms
someone who works for a business
An employee
A gardener.
a horticulturalist
demonologist
collector
Mason
A person who works with gold is called a goldsmith.
The skin is not any thicker.. A person who works out has bigger muscles verses a person who does not work out
Chemist
A co-worker. or colleague