First post of ####2017! I planned to write at least one post over my winter break, when I wasn't swamped with memorizing the course of the trigeminal nerve through the skull or any other such delights. However, seeing family and friends, playing an unhealthy amount of Skyrim (I know, I'm 5 years late on that one) and hitting some nice personal bests in lifting ended up taking priority. Now that break's over, I'm again inundated with wondrous topics such as acidemias and alkalemias, but I figured I'd make a quick, light post so that people don't think I've given up on this blog completely.
The simple answer to the title question is that you should only bring what you need. Your needs will vary based on your goals and what you actually do in the gym.
Here's what I keep in my gym bag:
Chalk: Mostly for deadlifts, although I recently started using it for benching too. I also bring some liquid chalk, because one of my gyms FOR SOME REASON banned chalk. Using chalk isn't cheating; not using it puts you at a disadvantage.
Belt: For my heaviest sets of squats, front squats, and deadlifts
Wrist Wraps: For heavy benching and squatting
Weightlifting shoes: For when I want to do bodybuilding-style quad work. The elevated heel helps emphasize the quads in squats, hack squats, etc. Recently, though, since I'm training for a powerlifting meet and only care about moving heavy ass weights, I've been wearing my Adidas Sambas exclusively. I find these to be the most versatile shoe for the gym, and I use them for squatting, benching, and even deadlifting.
Straps: Useful for when the skin on your hands won't permit you to continue deadlifting, but you still want to pull. Also good for heavy barbell rows, where using a mixed grip is quite ill-advised.
Headphones: Because sadly, not every gym plays metal
Lock: To keep my shit safe
I think that this is the absolute most that you need to bring to the gym unless you are a geared powerlifter (or if you go raw with knee wraps). If you're only interested in looking better, building some muscle, losing some fat, getting better conditioned, and/or getting stronger (without a desire to compete), then you can get by with no gym bag at all. Absolutely no supportive equipment is required to do these things other than a good shoe. If, by some chance, your gym doesn't require shoes, those aren't needed either, but you may want to invest in some antifungal spray.
If you want to use equipment, that's absolutely fine (as I said, I often do), but make sure not to use it as a crutch. Don't be that guy who wears a belt around the gym to do curls and machine work, or who benches 250 with wrist wraps and barely has the wrist strength for 215 without wraps. Additionally, if you need to wrap yourself up like a mummy just to get through a workout without severe pain, perhaps you should consider getting yourself healthy before being a gym warrior and messing yourself up further. Regardless of your equipment usage habits, I'd always recommend bringing a lock so that you don't have to carry your wallet and keys around with you, and headphones as well (unless you really like Maroon 5).
Honorable mention items: These are things I don't carry or don't carry anymore, but have definite value in certain situations.
Shaker: If you take a fancy pre-workout (if so, stop wasting your money please), an intra-workout, or a post-workout shake, this is probably a good idea. I just take my post-shake at home nowadays and drink from the water fountain while I'm at the gym. This may begin to change, though, because for Christmas I received the world's only cooler within a cooler, that holds your preworkout, a gallon of ice water, AND your post-workout shake.
Dip belt: I have one of these, and they're great for weighted dips and pull-ups of all types. The only reason I stopped bringing it is that gyms tend to have dip belts on hand now. While the Spud Inc belt has a much sturdier chain than those at the gym, the extra weight in my bag wasn't worth it for a piece of equipment that was already there.
Running shoes: If you like running on a treadmill at the gym, then these are a must. I personally despise running on treadmills and can think of thousands of things I'd rather do (such as running outside in the winter), but this is definitely worth a mention because running in lifting shoes is usually a terrible idea.
Resistance Bands: Many gyms don't have these, and they're a good tool for many exercises and warm-ups.
Showering supplies: If you shower at the gym, you'll need soap and a towel at minimum.
