Depends on the gym. Some do not allow it at all unless you sign up for some type of membership - or they tell you to do a free trial, take your billing info, and hope you forget to cancel.

The alternative I've commonly seen is they do offer a day pass, but it's basically the cost of an entire month to go even one time, while also making it extremely inconvenient by having to sign a bunch of forms every single time you go. This makes it so nobody except maybe a tourist/non-local would ever consider this option.