k and g sound completely different
 

k pronunce ku. ka ki ku ke ko..i dont understand properly what you are talking about but g can be ga gi gu ge go , we call these "onomatopoeic" of k sound.
we divide k and g properly. got it?:)

No, we don't. Those are just syllables.

Onomatopoeias are words such as どんどん、きゃんきゃん、パラパラ、グングン, etc.

Not "onomatopoeia." がぎぐげご are the [i]voiced[/b] versions of かきくけこ, which are the voiceless velar plosives. Although actually がぎぐげご could be velar fricative or velar nasal rather than a plosive in some dialects for some words. I actually speak like that because I picked it up somewhere when I was in the US before I went to Japan.