Yes, All visitors and contractors that enter your GMP areas should wear hairnets/beardnets as applicable based on your risk analysis I'm sure you've done.
Otherwise the requirements are based on your perceived risk to the product. I've seen places where all visitors/contractors had to wear lab coats or disposable smocks and I've seen places where only visitors had to wear those and contractors were allowed to wear work uniforms as long as they were visibly clean.
Typically all visitors/contractors had to wear close toed shoes and pants or jeans, no shorts. Things of that nature.
You have to have a meeting with your food safety team and discuss what you want then communicate/train your visitors/contractors on what is expected. This can be just telling them what to wear and then having them read and sign off on a visitor/contractor GMP acknowledgement sheet when they arrive at the plant when they sign in (the should have to sign in and show ID so you know they are who they say they are dependent on your site security policy and as applicable by your food safety standard.)