Bet what he's doing is in some grey area that the law and regs haven't entirely caught up with yet. I notice from his company's website, Gu is listed as CEO or Chief Executive Officer, not as a medical officer - in fact, the website says the company (CoolQuit) is currently looking to hire a Chief Medical Officer.

It's not clear if Gu himself is providing the telemedicine consults; I'd imagine he doesn't as talking to anonymous patients about their UTIs, sinus headaches and other such would probably be beneath him (after all, he was training to be a cutting-edge surgeon doing experimental work on transplants involving human fetal tissue). And doing telemedicine would take him away from his day job on Twitter! BTW, CoolQuit says on its site it's looking hire a lot of other medical professionals who can provide patient services; I'd imagine applicants would have to have the appropriate licensing for their their degrees, be they medical or nursing degrees, from at least one state.

On the general topic of MDs without licenses to practice medicine: many MDs in the US are not licensed health care providers, so they can't legally provide direct patient care. A number of MDs stop their medical training when they graduate from med school and get their MD degrees - they don't go on to do internships and residencies; some, in fact, get degrees in other fields like law, business and public health. Usually, these people work on the business, management and legal sides of the industrial health care complex - though many also do research in labs, or are academics who teach medicine but don't practice it.

Even when previously-licensed doctors lose their licenses for some kind of professional misconduct, settlement terms often allow them to stay involved in the field of medicine. Some even are involved in running medical practices, which they can do legally so long as they don't see patients, review charts or provide any input into individual patient care.

As for telemedicine and licensing specifically: all the laws and regs in the US are currently in flux, with the situation subject to change almost daily. It used to be, many states required that MDs providing care to patients within a particular state must be licensed directly by the state - whether the care was provided in person or over the phone or web.

But since the telemedicine industry came on the scene and has begun to burgeon, it has gradually been chipping away at the old way of licensing, state by state. Then this year COVID-19 hit, and the move to change the state regs of the past accelerated for two reasons: to allow MDs and nurses to travel to other states hit hard by the pandemic to provide hands-on patient care; to allow people stuck at home under lockdown orders in many states easier, wider access to medical care by telemedicine. The state I live used to require that all telemedicine practitioners providing consults to people within the state had to be licensed by my state or by one of the two states directly adjacent with equivalent licensing standards. But now those regs have been suspended by an order of the governor, and telemedicine companies can match up patients from my state with licensed HCPs in any other state as well as the territories and DoC. This seems to be the case across the US:

https://www.cchpca.org/telehealth-policy/cross-state-licensing