How to use SoundFonts in GarageBand 10.2?
Ah yes, this is something Apple's been sent feedback about in terms of their user interface design for GarageBand 10. It's actually a quick fix that will hopefully get addressed in an upcoming update (changing the default setting on installation will probably do the trick for most people).

Using SoundFont (SF2) files in GarageBand v10

To get your SoundFont files to work in GarageBand 10, follow these steps:


Launch GarageBand and open your project
Go to GarageBand > Preferences
Select the Audio/MIDI tab
Within the Effects section, tick the Enable Audio Units checkbox
Exit Preferences
If it's not already, open the Smart Controls window
Now click on the Plug-ins drop down menu and select Instrument > AU Instruments > Apple > DLSMusicDevice > Stereo (see screenshot below):





A DSLMusicDevice window will open (this may take a little while)
Now click on the Sound Bank drop-down menu to select your SF2 file (see image below):





Once you've selected your file, close the DSLMusicDevice window
Now press commandK to open the Musical Typing keyboard (or you can just go to Window > Show Musical Typing).


Now you can play the sounds from the SoundFont (SF2) file you selected.



Installing SoundFont files on macOS

The steps above show how to use SoundFont files within GarageBand 10. However, if you want to download/install additional SoundFont files in macOS to make them available to GarageBand and other apps, follow these steps:


Download  the SoundFont files you want
Within the Finder, press and hold the option key
Keep the option key down and click on Go > Library
This will open your user's Library folder (you can let go of the option key now)
Within the Library folder navigate to Audio > Sounds > Banks
Press shiftcommandN to create a new folder
Move your SoundFont (SF2) files into this folder


Now your SoundFont files will be available to any apps that support them.