someone on the mailinglist of rubycocoa helped me:

		attribs = [OSX::NSOpenGLPFADoubleBuffer,
		 OSX::NSOpenGLPFAAccelerated,
		 OSX::NSOpenGLPFAColorSize, BITS_PER_PIXEL,
		 OSX::NSOpenGLPFADepthSize, DEPTH_SIZE, 0].pack("i*")

		nsglFormat = OSX::NSOpenGLPixelFormat.alloc.initWithAttributes(
attribs )

that was the solution.