I have always spelled it out in my head, and the first time I heard someone say it I was utterly confused as to what they meant. To me, if I were to try to pronounce it, it would sound like "guy", as in the first syllable of "guide". Unlike Nasa or scuba, there's no universally obvious pronunciation for GUI—and thus I think it should be spelled out.
I have not changed my pronunciation or any of my own opinions based on this video; I only listed lessons to be learned from watching it in its entirety.
I always thought acronyms were pronounced phonetically whenever possible, like NASA, SCUBA, etc.
Even if that's not the case, "gooey" has become ubiquitous enough to be the colloquial pronunciation.