Hi Alan,
Take a look at
https://github.com/drowe67/freedv-gui/actions. This has a list of automated builds (most recent on top). Click on the one corresponding to the platform you're interested in to view more information. For instance, you'll want one of the Linux ones for the Flex AppImage.
Once you click through to the following page, you'll want to go to the Artifacts section on the bottom. For example:
Click on the download link to the right of the file you're interested in (e.g. for the Flex AppImage, you'll want the one named FreeDV-FlexRadio-...-aarch64.AppImage for the Raspberry Pi, or the one with "x86_64" in the name if you're running it on a regular Linux PC). This will download a ZIP file that you'll need to unzip to get the actual AppImage/installer/etc. Hope this helps!
Thanks,
-Mooneer K6AQ