You can open Chimera by ctrl-clicking on the Chimera icon and choosing "Open" from the menu that appears. Then it will let you start Chimera. After you start it once it won't ask again.
In macOS Catalina which came out a few weeks ago Apple now requires that applications be signed and notarized in order to open without the warning message you saw. In the past it required only signing which we have been doing for Chimera for several years. We are working of figuring out how to notarize the app and hope to have it notarized in the next few weeks.