Many people have met Gao Yang, but Sergei is different; Sergei is a Russian operative.
Being watched by any country's intelligence agency is not good. Gao Yang can deal with arms dealers, he can deal with information brokers, but he absolutely cannot deal directly with any country's intelligence agency.
The reason is simple: Sergei knowing about Gao Yang's existence means the GRU knows about Gao Yang's existence, which is bad but acceptable. However, it's unacceptable that Sergei knowing what Gao Yang looks like means the GRU knows what Gao Yang looks like.
For well-trained operatives, once they've seen someone, even without a camera or video recorder, they can still reconstruct Gao Yang's appearance with high accuracy.
And with the GRU knowing Gao Yang's appearance, it means Russian intelligence agencies know what Gao Yang looks like.
There's no problem now, but that doesn't mean there won't be any problems in the future.
