Someone rich spending a lot of money to obtain tutoring will make their score higher if they have any kind of aptitude. Likewise if they have easy access to books, extra study resources, a quiet space for study, no family distractions or challenges, and so on.

Poor people typically have none of those extra resources. Some poor people with extreme talent will be able to overcome the challenges of relative poverty, but others with equal talent won't.

It's extremely hard to create a true meritocratic system, and Gaokao certainly isn't it.