What is the cathode of a galvanic cell made with magnesium and gold?
The cathode of a spontaneous electrochemical cell is the cell in which metal cations undergo reduction. The electrons are supplied by the anode.
Without much thought, we can conclude magnesium is much less favorably reduced than gold. If you're familiar with its violent reactions in some strong acids, for instance, you can relate.
Quantitatively, we may imagine a reaction,
where the cell is the galvanic cell you may be describing.
Data from Colorado State.