It may be a corruption of “God’s Friday,” as noted in this link from the Global Board of Discipleship website.
Here are some key points from that article:
The source of our term for the Friday before Easter, “Good Friday,” is not clear. It may be a corruption of the English phrase “God’s Friday,” according to Professor Laurence Hull Stookey in Calendar: Christ’s Time for the Church(p. 96). It is the common name for the day among English- and Dutch-speaking people. It is a day that proclaims God’s purpose of loving and redeeming the world through the cross of our Lord, Jesus Christ. It is a day that is good because God was drawing the world to God’s self in Christ. As seen in John’s gospel, particularly, God was in control. God was not making the best of a bad situation, but was working out God’s intention for the world — winning salvation for all people. We call it “good” because we look backward at the crucifixion through the lens of Easter!
“Good Friday” is not a universal name for the day. The liturgical title for the day in the Western church was “Friday of Preparation,” since the time Jews used the word paraskeue (getting ready) for Friday, meaning the “day of preparation.” Popular names for the day are “Holy Friday” among the Latin nations, “Great Friday” among the Slavic peoples, “Friday of Mourning” in Germany, “Long Friday” in Norway, and “Holy Friday” (Viernes Santo) among Hispanic peoples.