The mistake was very natural, as the resulting sense was much more obviously appropriate than that of 'good tidings' for a word which was chiefly known as the name of a sacred book or of a portion of the liturgy. [OED]The word passed early from English to continental Germanic languages in forms that clearly indicate the first element had shifted to "God," such as Old Saxon godspell, Old High German gotspell, Old Norse goðspiall. Used of anything as true as the Gospel from mid-13c.; as "any doctrine maintained as of exclusive importance" from 1650s. As an adjective from 1640s. Gospel music is by 1955. Gospel-gossip was Addison's word ("Spectator," 1711) for "one who is always talking of sermons, texts, etc."