Gamecube too bright with elgato

Gamecube CDs have the same diameter as MiniCD/DVD (which is 80mm).

A sleeve for MiniCD/DVD should also fit a Gamecube CD.

Soruces:

Wikipedia MiniDVD

Wikipedia Nintendo optical discs

Unfortunately the answer is going to be the same. Your Macbook may have an HDMI port, but it's an HDMI Output, not an Input. The monitor is connected internally, so you don't have direct access to the monitor's inputs.

This is the way laptops (and iMacs) are designed. The HDMI port you mentioned is for multi-monitor setups or for outputting to a projector - it is an HDMI Out. The computer can send a signal out through that port to another monitor, but it can't take an incoming signal and transfer it to the laptop monitor.