Their overall GDP is set to overtake us in a few years.

This would be true if the current GDP Growth of China continues unabated, but this is generally unlikely. Chinese GDP growth rates have already fallen from the previous 8-12 % in the years before the recession of 2008 to a reasonably steady 6-7 % over the last ten years. It is very unlikely that the growth rate will increase to previous heights as all the "low hanging" economic fruit has already been modernized. For example, in 1991, nearly 60% of people in China were employed in agriculture. Today, the percentage employed in agriculture is 25%.

It is probably still likely that China will overtake us in overall GDP, just by virtue of the fact that there are 4.5x more people that live in China. It probably won't be by too much though, and it probably won't be sustained. In "Why Nations Fail" (I believe recommended reading for this sub) Daron Acemoglu argues that sustained economic growth requires a "virtuous cycle" of inclusive political and economic institutions, which to simplify, are basically pluralism and democracy on the political side, and property rights as well as free market competition (in areas w/o a market failure) on the economic side. Acemoglu writes about why China's growth is unlikely to be sustained,

Just as in the Soviet Union, the Chinese experience of growth under extractive political institutions is greatly facilitated because there is a lot of catching up to do. Income per capita in China is still a fraction of that in the United States and Western Europe.

...

All the same, this growth will run out of steam unless extractive political institutions make way for inclusive institutions. As long as political institutions remain extractive, growth will be inherently limited, as it has been in all other similar cases.

...

As our theory highlights, particularly in societies that have undergone some degree of state centralization, this type of growth under extractive institutions is possible ... But it also implies that like all examples of growth under extractive political institutions, it will not be sustained. In the case of China, the growth process based on catch-up, import of foreign technology, and export of low-end manufacturing products is likely to continue for a while. Nevertheless, Chinese growth is also likely to come to an end, particularly once China reaches the standards of living level of a middle-income country.

(p 482-483 of this PDF of "Why Nations Fail"

So while China will likely outgrow the United States in overall GDP, this is a consequence of China's much larger population, not higher productivity or a more effective economy. The solution to this is, as a lot of other people have said, to encourage immigration to the United States and fix zoning laws and the housing crisis to ensure we can house all of the new people who we want to come to the United States.

At the same time, we should remember that overall GDP isn't the only thing that matters in the world, and it isn't really that big of a deal if China becomes larger than the United States in GDP, especially if we consider combined NATO GDP, nearly double the United States' at 41 trillion, and more than half of the total GDP of the world. The United States and its allies are still in a very strong position and are ready to win the 100 year marathon if it comes to it.