Christian music is a music that reflects the good values and morals of human beings, often categorized as edifying music. Music that causes you to think positively, do, or desire to do things inline with the Christian value system. It reflects God's good nature, values and morals.
Don't be fooled by people expecting the word of God to come up in every lyric for it to be Christian and the lack of a religious word in the song makes it secular. That is just based upon many Christians misunderstanding between church music and Christian music.
Christian music is a branch of the music industry which includes many genres of music. For hundreds of years, Christians have been writing music in all types of genres with a Christian message. Generally, Christian music can exist in any type or genre! Every good thing comes from God.
Gospel music is different because it is solely about good news. What makes Gospel music is the message. Gospel music is not always happy. Sometimes it is sad, but with understanding that it will be better. Gospel music is a personal testimony, that restate the word of God, and his many promises to his people.