Yang was a naturalized American citizen. He lost his Chinese citizenship at the time of his naturalization. He then had to renounce his American citizenship to reacquire Chinese citizenship.
Assuming one of Gu's parents was a Chinese citizen at the time of her birth, then she always has been a Chinese citizen. She didn't have to acquire Chinese citizenship, which would have required her to renounce her American citizenship. According to Chinese law, she has always been a Chinese citizen and never an American citizen.
As I said above, if someone was born in a foreign country and hence automatically acquired foreign citizenship like in the US, she/he has a chance when becoming an adult (21?) to select to become a Chinese citizen on her/his own will. This is assuming, of course, at least one parent was a Chinese citizen.
Now, back to the core issue discussed here:
whether she can be a dual citizen at this point. The answer is an absolute No. In both theory (law) and practice. Again I'm talking about legally which we should assume that's the case for a celebrity like her now. Just in case that you or someone still thinks there is a distinctive possibility or grey area that China in this case would pretend she would have renounced her US citizenship (benign negligence) and doesn't care, like the US practice effectively. The answer is still No. China strictly enforces this law. Heck, they even strictly enforce some policy on permanent resident or green card holders (like you have to give up your Hukou or something like that).
To be sure, there are Chinese citizens that effective violate the citizenship law by holding foreign passports without acknowledging it publicly or to the government. This is illegal and there have been crackdowns. In Eilleen Gu's case, I doubt that's the case. It's such a high-profile case and they will make sure she would provide the proof that she has renounced American citizenship first.
There is a similar case before about a guy from UK, he is called 华天. He participated in Equestrian representing China in 2008 Beijing Olympics.