Here is an adapter plate that allows you to mount/retrofit a Duet WiFi control board to a Geeetech G2S Pro Mini frame or in place of a GT2560 on other Geeetech printers, You'll need four M3*16mm cap screws (bag #21 in the Geeetech kit) with 8 washers (bag #5) and 4 M3 nuts (bag #9). You'll want to place washers on both sides to give the screw and nut something to grab onto while tightening. The Duet WiFi electronics are great and far superior to the stock GT2560!
Update 11-1-17: I uploaded a picture that shows how I stacked the Duex5 and Duet WiFi boards with the adapter plate on my Rostock 301.
FYI - Here's a link to a great crimper I found on Amazon to use for the Duet WiFi connectors: