They never mention it in the series, but I've always assumed it was short for George O Bluth Jr as well. When he does a magic show, they announce him as "A Magician called 'gob'", but everyone else says "jobe", which is how they tried to make his name unique, I suppose.xhonzi said:
But I think I learned someting last night- GOB is Jobe's initials? George O. Bluth, Jr.? Is Jobe an odd pronunciation of GOB? Does anyone ever actually call him GOB, or do you just see it in print? I don't think I had caught on that Jobe's name was really George Jr.
Of course, it does make his name George O Bluth Bluth, when they call him "GOB Bluth". Which may be part of the humor.
http://en.wikipedia.org/wiki/List_of_Arrested_Development_characters#Gob
We were right. Oh, and the "O" stands for "Oscar". Interesting...
And Oscar's full name is "Oscar George Bluth". I love this family!