The GPGTools team posted this on Twitter this morning:
"We are in the midst of preparing the release of GPG Suite for macOS Catalina. It should be available next week.
In order to continue using GPG Mail, please hold off from updating to Catalina for now.
We generally recommend to wait for .1 versions of macOS releases."
The explanation for needing to wait until a macOS version is released is here:
While a new major version of Mail.app may introduce only minor visual changes, the codebase underneath it changes dramatically. Other Mail applications provide a number of functions, plugin developers can leverage to extend the application. These functions are guaranteed not to be changed or if they are, developers are notified in advance in order to adapt their plugins to the changes. This however is not the case for Mail.app. Apple can and does change everything they want whenever they want to. There's no notification of what has changed whatsoever, not even for the developers of the most popular Mail plugins.
Especially during the beta phase of new major versions of macOS, Apple usually introduces a slew of major internal changes with each beta. So in the worst case, plugin developers have to reverse engineer Mail again and again in order to find new or modified functions they could use to enhance Mail the way they need to. In the worst case, those changes can require us to re-reverse engineer most of Mail.app and rewrite a whole lot of code.
In the interim, you should be able to use the decryption functionality in the Services menu to continue to decrypt your messages, just not automatically.