A gym bag should make your session easier, not serve as a collection of equipment you rarely use. Build it around the training you actually do and the rules of your facility. A small, predictable kit saves time when you are heading straight from work to the gym.
Pack for the planned session
Choose suitable training clothes, footwear appropriate to your activity, and any items required by the gym. Bring your workout record in whatever format you use. If you rely on a particular accessory, make sure you know how to use it and why it belongs in the session. More equipment does not automatically improve training.
Keep items ready to use
Separate clean and used clothing, check that closures work, and replace worn items when needed. Put small accessories in the same place each time so you are not searching between sets. Keep valuables and personal items away from lifting areas. A tidy bag can reduce the distractions that make an otherwise simple workout feel disorganized.
Remove what stays unused
Review the contents occasionally. If an item repeatedly comes home untouched, decide whether it needs to travel with you. Keep specialist equipment for the sessions where it is relevant. Preparation should make it easier to arrive and begin, while leaving your attention available for the workout instead of the contents of your locker.