Peach is a very subtle flavor in beer. I just added ~5 lbs of white peaches (pitted, sliced, and squished up a bit) to the secondary of a sour honey wheat beer. The sample I pulled after a week of fermentation had a noticeable peach aroma/flavor, but it was not over the top.
That works out to 2 lbs of ginger per bbl. A bbl is 6 times bigger than your 5 gallon batch, so you need 1/3 lb (5.33 oz). That is a lot more fresh ginger than I have heard of people using (Ive only tried dried ginger, and 1 oz was too much). I would start off at 1-2 oz, you can always add more if you need it.