It's likely that her mother guided her in this direction.

Notice how her last name is Gu, the same as her mother? As far as I know, women in the US usually don't get to pass down their last names on to the children. So for Gu to carry her mother’s last name, her American father was likely absent from her life since a very early stage, and likely had little to do with her up bringing.

This meant that Gu’s education was probably single-handedly left to her mother’s family. Now Gu’s mother was a Chinese born and educated scientist and ski coach, and had decided early on that her daughter should keep her Chinese roots, which is quite evident from the fluent Chinese she speaks, at a level that puts many second generation Chinese immigrant in the US to shame.

So we have a Chinese emigrant mother that cherishs her Chinese identity, likely having left China due to some very personal reasons, abandoned by her American husband and having to endure the hardship of bringing up her daughter alone on foreign soil, likely suffering quite a few racist attacks along the way which made her identify quite a bit more as Chinese, shown in her participation in the activities of Chinese community in California. What would she do when she finds out that her daughter has talent that her home country would cherish?

She'd pull some old contacts and deliver this prized kid to her beloved motherland like any good old Chinese citizen would. Where she failed to make a difference in China, her daughter will. Where she didn’t feel at home in the USA, with her daughter she may in China.

Sponsorship and stuff are really an afterthought. We all know that American sports, fashion, and advertisement industries are all vastly bigger than China’s. Gu ran very real risk of losing big money by announcing her choice of China over the US in 2019, at the height of Trump’s US vs China rhetoric. Not to mention that Gu already expressed her wish to compete for China instead of the US as early as 2013, when she was only 10 years old. What does a 10 year old know about which team to play for? Of course it’s all drilled in by her family.

Above: Gu Eileen’s weibo post “Thank you all. But working hard toward the 34 provinces rather than the 50 states.”13–8–18 (Chinese writing arrange the dates on a year-month-day format)

So let's cut the lame “for money” excuse. Gu skies for China because of the Chinese education she received at home. And this wasn’t an opportunistic choice, it was a tiger mom’s long term plan years in the making.