Many, if not most, Biblical scholars advance a theory that the apostle Paul did not have access to any of the Gospels, at least how we know them today. Bart Ehrman states this explicitly:

Paul really did not know that there were any written Gospels, since in his day, they hadn’t been written yet. This includes Luke, probably written around 80-85 CE, some 15 or 20 years after Paul’s death.

This makes sense, being as most scholars date the four gospels to well after Paul’s death (64 AD). But the interesting thing is that Paul gives a fleeting reference to Luke:

1Ti 5:18 For the scripture saith, Thou shalt not muzzle the ox that treadeth out the corn. And, The labourer is worthy of his reward.

The statement that a laborer is worthy of his reward is not an Old Testament teaching, but a teaching unique to Christ:

Luk 10:7 And in the same house remain, eating and drinking such things as they give: for the labourer is worthy of his hire. Go not from house to house.

Of course, some might claim Paul was quoting from a pre-gospel document or floating sayings of Christ. But what is more likely, that Paul is quoting the sayings recorded in a Gospel by his historian travel companion, or that Paul is quoting a never-referred-in-antiquity source documents or floating sayings? The reader can judge.