Quoting Frank Warmerdam, the maintainer:
I pronounce it "goodle". I had originally thought to call it the
"Geospatial Object Oriented Data Abstraction Library" (GOODAL)
to make the right sound obvious, but I was too lazy to type GOODAL
all the time, so I dropped the OO part. Some folks might say I
dropped it from more than the name. :-)
Most folks pronounce the library "gee-dal" which is ok too but not
FWIW, I find the "goodle" pronunciation gets misheard more often than not, and use the one that spontaneously comes to mind from the spelling.
Edit: As Ragi pointed out in the comments, Frank himself switched away from his original pronunciation during the time he was working at Google. Personally, I've heard people close to the project use the "goodle" pronunciation.