If all they need is simple code programming from the keypad, I've used a Speco piece a few times. It has the relay unit included so all you need to add is the doorstrike and power supply. It's simple and inexpensive, but may not be a good fit (only up to 130 user codes).
[Link: specotech.com]Similar devices are available from Viking, Doorking, Corby, etc.
If it's a gym in a commercial building (for employees only), this might be okay. If the business itself is a gym, they're probably going to want a more full features system that can be administered from a computer.
I installed cameras for a gym recently where they had a doorlock with keyfob & numeric keypad access. It was connected wirelessly to a computer in the office that made adding and removing codes easy. Not sure what the product was, but here's a picture of the unit... maybe someone else will recognize it?