It is my opinion that the Holy Spirit shortens the Turk’s name and does not only call him “Magog,” which is the correct, full name in Genesis 10 [:2], but He breaks off his head, takes away the first letters, and calls him “Gog,” though both “Gog” and “Magog” are a single name in this passage [Ez 38] and in Revelation, and both also signify the same Turk. He does this to give us comfort, by showing that He is a bitter enemy to Gog, just as any man calls his enemy by a nickname if he wants to show his anger or displeasure, as when one otherwise calls someone “Johannes” on a daily basis and out of anger shouts “Hans!” – Martin Luther
Luther identified the Ottomans as Gog simply because he found it expedient, not because he had any exegetical basis to do so. And that, dear Martin, is eisegesis.
Luther was an often witty theologian but his exegesis was frequently really terrible. As here in his comment on Ezekiel 38.