I've studied Eschatology for over 30 years and have never heard that the Celts are considered Gog and Magog. I've heard some who have considered Gomer to be the Germanic tribes, but frankly I think that's incorrect. Where did you get the idea that the Celts are Gog and Magog?
http://www.soundchristian.com/man/
Gog and Magog were big in Celtic mythology, and there are many who consider the Celts the sons of one of or the other (remember, Magog was a son of Japheth). A friend of mine from Ireland first told me that the Celtic people were from the tribe of Magog, and there is some
We in the US often think of such things as “Places” when the ancients would see them as “People”.
http://en.wikipedia.org/wiki/Gog_and_Magog