I've attempted to import GameFlow with Unity's package manager and I receive an error: "ERROR: unable to locate the GameFlow .bak files for Unity 2020.1.  Please reimport the GameFlow package."  I try re-importing, but nothing happens.  I have noticed that under the "versions" folder there appear to be versions for 2019.1, 2019.2 and 2019.3, but no other versions.  

Unfortunately this makes the tool unusable.   Please help and thank you in advance!

you need to duplicate the folder "2019.3", which is located in "GameFlow / Versions / 2019.3".
now rename the folder to "2020.1".
now go to the menu "Tools / GameFlow / Reinstall"  and that should be enough ...