The gatekeeping happens at the end or after the university and before you enter the profession.

It's generally called a test (or multiple tests) which judge the quality of one's knowledge before one is allowed to practice as an expert in a certain area.

The graduation is the part where the University produces a certificate in which they state that they have indeed tested somebody's knowledge and how good it was determined to be. If a person goes through the whole learning process but don't get that certificate, future employeers might not (in some areas, they legally can't) consider that person for employment in that area (I explain why at the end).

Generally the actual learning is not gatekept: for example, in my area - software development - people absolutelly can do the entire learning outside formal education and still end up working in it professionally, though at the start of one's career one still has to have some kind of evidence of one's capabilities (which in this case isn't provided by a University having assessed your knowledge on it), so normally the path to it that bypasses Academia involves first working professionally in an adjacent area (such as systems administration) and moving from that to software development (good sys admins have to know how to program)

However for "protected professions" (such as Law) or for were the costs of errors can mean death (such as Medical or Civil Pilot) at minimum you have to be assessed including a significant practical period under supervisions (a couple of years for a Medical doctor depending on speciality, 1000h of flight for Civil Pilots, plus specific training each kind of plane they're flying) and that practicing under supervision is lot harded - often impossible - to get if a person didn't come via a formal education setting.

Also in some areas it's pretty close to impossible to get certified as knowledgeable without going through the entire formal Education process, which is indeed unfair and should not be the case - if should be possible for anybody to pay to be assessed and certified without having to pay for the formal learning.

Even in areas which are neither protected professions nor life-and-death, not having the certification which is the Diploma negativelly impacts a person's chances to find their first and maybe second jobs. The problem is when hiring managers get lots of candidates for a position, they don't have time to talk to them all because they also have to do their normal job alongside candidate selection, so instead they prune the list of candidates and not having something that in some way certificates that a candidate has the required knowledge (which for a first job is generally a Diploma, but for latter jobs is going to be previous job experience) is a common criteria because it usually works.