The theories of 'gniibe'
There are several theories for the name 'gniibe'.
Sometimes gniibe claimed that 'g新' is a new (GNU) Kanji character for GNU. He even tried to register it to Unicode consortium.
g新部 means GNU People in this theory.
A variant of 新
In Kanji characters, there are many variant of glyphs.
For example, you can see different glyph for '新' in the logo of Asahi Shinbun. You can find another stroke between the upper half of '立' and the lower half of '木'.
Once, gniibe claimed that his '新' is special and it has another stroke. It looks like g新, when using computer.
Milk Cap King
Gniibe said he was a chairman of the student board in Shikishima Elementary School in 1978-1979.
He said that the major reason why he could become the chairman was that he governed the school by the power of milk caps. At that time, Shikishima students (especially, boy students) collected milk caps, and caps worked as a kind of currency.
Because he had thousands of caps including many kinds of caps from various prefectures in Japan, classic milk caps of 60's, un-published milk caps, etc., his power was tremendous. Also, he invented games using milk caps such that player could get a valuable cap when he won (but hardly), and collected more.
In Japanese, milk is written as '牛乳' and it is pronounced Gyu-u-nyu-u.
In this theory, 'gniibe' is considered the "Milk Cap King", and pronounced Gyu-u-nyu-u-be.
Niibes in Gunma (群馬) region
It seems that origin of Niibes is Niigata. Given name of Gniibe's father of grand-grand father's is 三四郎 (Sanshirou) and he or his son 政右衛門 (Masaemon) came to Kanto area.
Gniibe could mean "Gunma" Niibes (instead of Niibes in Niigata) to distinguish different families.