I'm often driven crazy by the lack of kanji in manga and other written works. I spent so long learning their Asian hieroglyphics, the least they could do is use them!
Anyway, I believe the sentence, filled out, is:
なにするの -> なにすん (の here makes it a question)
だ -> じゃ
この -> こん
ガキは -> ガキャーーー
The ーーー at the end is just an extension of the last vowel, probably because the speaker is screaming in the middle of a typically histrionic manga attack on someone else.
The translation would then be something like:
What the hell are you doing you fucking brat!
... where "brat" could be swapped out with things like "punk" or other variants. You could also vary up your choice of curse words and a little bit of the order, in order to best capture the feeling you think the manga is trying to convey.
I think the reason situations like this 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. That's why you see it a lot in manga.
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?".
Note I'm not at all a professional translator.