I have downloaded the Swiss Covid app, which was released in Switzerland on June 26. GAEN API exists thanks to the work of the two Swiss Federal Institutes of Technology in Lausanne and Zurich. They came up the the design for a decentralised privacy preserving proximity tracing app using Bluetooth and met, first with Apple, in an effort to obtain permission to obtain chip-level control of Bluetooth Low Energy, so that the app could operate in the background. Apple at the time was working on its own tracing app, and was wrestling with determining proximity without using GPS, which the DP-3T group had already solved. Apple wanted to using the design in its own app, but the group persisted in demanding only an API, arguing that every country, and, in larger countries, every jurisdiction had different requirement. Once Apple was amenable, Google was approached. It, too, was working on its own app. Long story short: The documentation and Open Source code for the Swiss app, including prototypes and testing modules, can be found on github/DP-3T.
The app does not require overwhelming adoption. The adoption could be overwhelming in certain settings and still suffice to be useful, e. g., if most people who commute by public transportation adopted it.