The nintendo official sdk of gbc is available?

gbdk is not good. have any other choice?

"Not good" in what way? And what would make the official SDK any better?

I have only seen leaks of the official GBC SDK, not spefically the DMG SDK (assuming it exists?), for what it's worth. I have no idea what is inside it though.

What's wrong with GBZ80 assembly?

List of assemblers for Game Boy: http://gbdev.gg8.se/wiki/articles/Assemblers

The gbdk C compiler is very...inefficient, let's say. Its use is usually recommended to not go outside the bounds of a hello world example.

As for the assembler that it comes with...wait, does it? As if so then I'm gonna have to try that out.

Now, something tells me the official SDK really isn't all that much better though. I wouldn't know, but if it's really been leaked then perhaps I could find out.

GBDK is based on a decade-old snapshot of SDCC. Hopefully a newer release would work better...