why gnu start with g AND
why I ask this question

And why not?

It's got to start with something and a 'G' seems as good as any other
letter to make a pronounceable word.

So, why did you ask this Question?

Curiosity!

The very same reason for hundreds of other questions.

Maybe a gnative of Gnamibia first gnoticed it and gnaively gnamed it
"gnu" in Hottentot because it seemed like a gnatural gname for this
fign "gnu" agnimal.

You may find these websites helpful:

http://www.acronymfinder.com/af-query.asp?p=dict&String=exact&Acronym=GNU

http://dictionary.reference.com/search?q=gnu

Are we talking about the animal or something else called gnu?

GNU stands for "GNU's Not Unix". It one of those recursive acronyms
traditional to Unix naming. There are a number of these. GNU is not
the first.

- Hammer

There's a complicated answer to this that involves a lot of
linguistics jargon. Here's the easy answer: The first Europeans to
write this word had a classical education, and in Attic Greek, when n
preceeded a vowel that had a strong sound with the tongue depressed
and/or the mouth rounded, that sound was signaled by putting a g
before the n, e.g., gnat and gnome. If n preceeded a vowel that opened
the mouth wider and raised the tongue a bit, that was signaleed by a
preceeding k, e.g., knack, knee, and knife (kno- words, like knot, and
knu- words, like knuckle, used to be pronounced a bit differently).