Ah ok, is there a way to get the exact coordinates? As the coordinates I used I copy and pasted from AFX.
Well, it depends what you copied from...AFX will indicate coordinates for the Southern hemisphere starting with an S, but GSX wants a minus sign, so you simply must be a bit careful here.
However, even adding the minus sign, the coordinates were still well outside from the airport, about 24 miles out.
Note that, you cannot custom-pushback whenever you like. The coordinates must be those of an existing node, so GSX will take the closest on it finds to that coordinates. This means, the best way to do it, is to double click on a node, and copy coordinates from there.