I was fortunate enough not having to take the test (coming to the States when I was a Freshman in high school). However, I watched my sister going through it ( I was 12 at the time). It was horrifying. Like any loving sibling, I took the opportunity and made sure that I messed with her as much as I could
And to this day, she still uses my behavior during the time as an example of why one should never have younger brothers
In addition to my sister, my wife also took the test (she came to the US after her freshman year in college). Her mom was a high school chemistry teacher for decades (in charge of senior classes for most of her tenure) before she retired.
So some of my conclusions based on my observation of my sister and stories from my wife and her mom.
1) Gaokao is not SAT. Unlike the SAT with math and English, Gaokao is usually consist of many subjects, which varies from time to time and province to province. It still confuses me to this day. So I'm not going to go into the details. However, Chinese, English, math, physics, chemistry, biology seem to be the common subjects.
Because of the number of subjects involved, the test usually takes many days, from 3 to 5 days ( usually 2 subjects each day). Again, I have no clue of the exact length now...
2). Gaokao is much much harder. Unlike SAT, which tests base line aptitude (I took SAT and did pretty well), Gaokao, in my understanding, is most likely designed to challenge your upper limit in knowledge and problem solving skills.
This is also why it is so taunting for the students. The good comparison would be the Hell's week at the end of the Navy SEAL recruitment. All the questions on the tests have been designed to find and challenge your limit. all that emotion that you see in photos tells you how scared the students are.
3). The implications of the Gaokao is huge. Gaokao is not just a single test, but an accumulation of all your hard work in all 3 years of high school. One test and one test only. This one test decides your entire future. Colleges don't care how you have done in any of the previous tests that you have taken before this test. They only look at your combined score of your Gaokao. If you misfire, you are done!