Hi Astroman,
I understand that traditionally Gui is used to mediate between the abundant Metal (Geng's) with Yi. It's called mediating useful god 通關用神.
However... I'm afraid that's not the way it works in Mangpai. The intention of the chart is to control Wei, and Wei swaps image with Yi. Gui produces or strengthens Yi, so Gui is not auspicious.
On the other hand, Gui swaps image with Chou, so it's time for Chou to control Wei. This is auspicious.