GATS covers services. Like health insurance. The USA currently does not have many foreign health insurers in our market, because of the 50 different state markets, which make selling insurance here too complicated for foreign firms. (so they say) This is a longstanding issue and as trade barriers are prohibited by GATS, the EU has petitioned the US to provide some way firms can sell one "Harmonized" (or "harmonised") national policies, (like they do elsewhere, most notably the Third World countries).
What happens then?
When foreign insurers enter the US market, the provisions of GATS kick in to 'protect their investment'. The companies who participate, are in a sense, given a "right" to be here and an imaginary value is assigned to it based on its potential future worth, not their actual sales. If in the future, the US or a state does anything to limit their "rights" the US can be sued in a secretive, closed, WTO-court and the penalties can be huge.
BTW, US companies don't get compensated every time a law is made or changed.