I believe the sentence, as written in more conventional Japanese, would be:
なにすん = なにするの (の here makes it a question)
じゃ = だ
こん = この
ガキャーーー = ガキは
The ーーー at the end is just an extension of the last vowel, probably because the speaker is screaming in typically histrionic manga fashion.
The sentence means something like:
What the hell are you doing you fucking brat!
You could change up "brat" to "punk" or "asshole", or other things. I put in "fucking" because I felt that it was needed to convey the real sentiment of the sentence, but you would be right to point out that there is no direct equivalent in the original Japanese. My goal here was to give you a working understanding of what is happening to carry with you for general use, not to provide a perfect translation of this particular sentence.
I believe the reason dialogue in manga, like this one, are mostly done in kana and not kanji is because it is meant to reflect spoken speech (is that redundant?), not written text, and so it conveys inflections and pronunciations that would be hard to do with fixed kanji.
The only way to learn this kind of thing is with practise. The way people play with abbreviating words and changing the way they are said is by definition beyond textbooks. Think of it like how in English you could see "whassup?", or even "'sup", in a comic, and only by familiarity would you know it's actually "what's up?".