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There is of course the built in diopter adjustment but the manual mentions on page 46 that an optional diopter lens, type GFL can be used. This is inserted between the viewfinder lens and the rubber frame. Hope that helps.
I don't know of any other magnifier available such as I think Leica have for the M series cameras.
Due to the design of the eyepieces on the G system, the magnifier made for the C/Y SLR system will not fit. Even with some clever DIY I don't think it could be made to operate properly, although, if you could get hold of one, you could try to see if they would work by holding it to the eyepiece and adjusting the focussing mechanism on the magnifier.
If the DIY idea does not put you off.....any make of magnifier would be an option.