DeBleeder GarageBand FAQ
Logic and Garageband are only " Audio Units " which for no reason Apple calls "components". This format is exclusive to Apple and OSX, though a few 3rd party DAWs have support on OSX. So the only file that needs to be installed is either the Debleeder32.component or DeBleeder64.component. Probably the 64 one, especially if you are running Catalina which only supports 64 bit audio stuff now.
Anyway there is a FAQ on some issues you will face with some security changes to Catalina if you are running it https://wilkinsonaudio.com/
pages/debleeder-faq, also this video below which is actually for putting together an automator script (alternate method to fix security attribute) shows in the first 8 minutes exactly what you need to do to install the file in Logix which is the same exactly that you need to do for GarageBand. Worthwhile to understand what you need to do to install. Also you will need to reboot as the caches on osx rebuild on a reboot.
Video Link to a 10 minute example showing what is happening regarding security. May put you to sleep
Finding this fix below was the end of a very long process to find out what was going on and for the two people with GarageBand I had to help it was the fix.
sweetcare/articles/my-newly- installed-plug-in-isnt- showing-up-in-garageband/
A user forwarded this explanation below, this in this case is needed because he still had the com.apple.quarantine flag on the DeBleeder file. If you get rid of this either by using the Automator script routine shown in the main DeBleeder FAQ or the terminal commands it would not be needed, but useful information if you are just using the security preference pane method to deal with changes Apple made in their security.
The fix seemed to be to clear the cache as explained in the Sweetwater link, I had already tried this previously.
But then to restart and run Garageband whilst holding down the option key.
This stops Garageband opening a session and also when the security window appears with the options to either "delete" or "cancel" running DeBleeder, the hidden option to "open" is also available.