Soundfonts in GarageBand: where can we select the .sf2 files?
Try double clicking the picture next to it.

When you hover over the picture changes to three lines with dots on them so you know you're in the right place.

Then a dialog opens, where you can change the sound bank.

However, I'm having a problem at the moment that the latest soundfonts I downloaded are crashing GarageBand when I close the dialog, although one I already selected is OK.