If the word is used as a common noun, it is not capitalized. If it is part of the title of a work (i.e., the Gospel of John), then it is capitalized.
Other instances of which to be aware include referring to the four canonical Gospels (i.e., the ones in the New Testament). ‘Gospels’ is capitalized when referring to the four Gospels.
The other instance in which ‘Gospel’ is capitalized is when referring to the message of Jesus Christ and/or the Christian Church. In this case, it is a proper noun and is to be capitalized.
No, the only time it would be capitalized is when it refers to a specific gospel like the Gospel of Luke or the Gospel according to St. John or the Gospel of Mark or when you say the New Testament Gospels. But if you say, “That is the gospel truth,” it would not be capitalized.
If it’s referring to the Bible, I would capitalize it. I always capitalize a pronoun when it’s referring to God as well.