According to Griffith's valuation, which is over 50 years after your John Gaggin emigrated, there was a landlord named Christina Gaggin with property on Gaggin's Lane (off Military Road) in the parish of St Anne, Shandon in the Union of Cork in Co Cork.
There are a few other scattered Gaggins in Ballybane, Cloyne, Middleton; in Ballynamona, Kilmahon, Middleton; and Shanagarry South, also in Kilmahon.
The pickings are very slim for Gaggin. So I would not rule out Goggin in your search. Now Goggins, they are all over the place! http://www.askaboutireland.ie/griffith-valuation/index.xml