Disguise isn't ********. If putting on a disguise allowed you to go anywhere you wanted, then that's pure nonsense.
A person's face is naturally the most identifiable feature, but besides the face, there's so much more that can mark a person, like body posture, eyes, fingerprints, and so on, there's just so much.
However, disguise works for Gao Yang; as long as he doesn't attract special attention, and no one stops him for an iris scan or fingerprint check, he can easily get by. Why is that? Because Gao Yang trained for a long time under the FBI agents under Murphy, forcing himself to change his posture and body characteristics. Pair that with advanced makeup techniques, and he can avoid most standard identification methods.
Of course, if he attracts any attention, then no amount of disguise or hiding will work. As long as someone catches him and runs a test, everything would be revealed. But just not attracting attention is enough, completely sufficient.
