Paul's letters were written before any Gospels were written.
When he refers to the Gospel, he is referring to the good news from God. He is not referring to Scripture at all, nor even to any of his own epistles, because Galatians is the oldest book of the New Testament.
The inspiration of New Testament writings is explicitly asserted when Paul quotes the Gospel of Luke as Scripture (I Tim. 5:18), and when Peter refers to Paul's epistles as Scripture also (II Pet. 3:15, 16).