Open the Project Settings (File ➤ Project Setting ➤ Tuning...) and select the Tuning tab.
The first slider lets you set the the pitch of ALL Software Instrument sin your Project. Enter the 20 into the numeric field on the right

If you need to tune individual instruments, then you have to do it in the Instrument itself (like you did with the EXS24). If the instrument doesn't have that parameter, then you might be out of luck.
BTW, most of the "orignal" GarageBand Software Instruments are based on Logic instruments, just with a more simpler GUI. If you use an Instrument that is based on the EXS, then you can open that directly in the EXS24 Sampler and do the tuning offset there.
Hope that helps
Edgar Rothermich - LogicProGEM.com
(Author of the "Graphically Enhanced Manuals")
http://DingDingMusic.com/Manuals/
'I may receive some form of compensation, financial or otherwise, from my recommendation or link.'