Gatekeeping is a restriction on something or trying to restrict access to it. It's everywhere. Think of any meme in the format: "If you (do ____ or don't do ____) you are not a real ____!"
There are some forms of gatekeeping that are good or at the least not as bad as others. Think of things like prescriptions for painkillers or age restrictions on alcohol.
There are other types of gatekeeping that are not as good. Many related to gender. "You aren't a real woman unless you cook", "Aren't a man unless you build cars!", and the list goes on. These aren't enforced anywhere. They are just gender stereotypes built to exclude some set of people. These are just silly examples but they are so frequent online (and offline) that they probably aren't noticed that much by most people.
Trans people deal with a lot of gatekeeping in medical settings. Want to get hormones? Well, we need to have a psychologist test you for as long as they feel because they clearly know better. There are many horror stories of therapists denying proper treatment until they 'feel' the patient is ready. In some places there is the requirement to live full-time, without hormones, for a year before anything can be prescribed. Even if that isn't the standard practice it could be imposed on you by the therapist. Many have been told "You just aren't trying enough" if they don't conform to some stereotype. This isn't always the case. There are changes being made but not that fast and not everywhere.
This has affected me a lot lately because I want to get a surgery but I need to get two letters of approval, from separate therapists, before my chosen doctor will even schedule anything. I am jumping through the hoops but even then I have to hope that insurance doesn't add another hoop at the end that I wasn't aware of. Often times I feel resentment. Why should I have to get approval to do this? How many people elect to have an expensive, painful, and significantly life altering surgery on a whim? Even if they did just up and decide they wanted that they would still have to find a doctor to perform it, which I consider to be enough of a gate. I understand in some ways it's tied back to insurance and that they won't pay for things unless you play along.
I have been playing 'good girl' until I have my letters. All it takes is one asshole to claim I'm not clearing this hoop because I don't "bake enough cupcakes" or whatever to set me back and stall my life. It still leaves a bitter taste in my mouth that I have to waste my time and my money, even with insurance therapy costs money, to get approval. It seems that the system is set up as a discouragement or a punishment for being 'different'. I've heard that if you look at it, you can see that it's set up by the cis, for the cis. We need to think of the cis and make sure they don't get sucked into a surgery or transition they don't need! What if people explored themselves and realized they don't need to conform to what society wants (or maybe they decide they do but they did so willingly)? Won't somebody please thing of the cis!?
I don't have any solutions. I probably don't even have the entire story. It gets on my nerves that I am essentially being asked to "prove you're really trans" by paying for X and jumping through hoop Y. All this by people who don't really care because it's not going to affect them. In the end it just feels like the system (cis-tem) is trying to encourage me to repress and conform or just go away instead of helping improve my life.