Paul uses the word Apostle in a more general way than do the authors of the Gospels.
The word Apostle appears only eight times in the Gospels. Mark and Matthew each use the word only once. John does not use the word Apostle at all. Luke, who was a companion of and very close to Paul, uses the word six times, much more freely than do Matthew and Mark.
Mathew’s use of the word Apostle is specific only to the twelve chosen by Jesus.
The names of the twelve apostles are these: first, Simon, who is called Peter, and Andrew his brother; James the son of Zeb’edee, and John his brother; (Matthew 10:2)
Similarly, Mark’s reference is to the twelve who were sent out two by two to cure unclean spirits.
*And he called to him the twelve, and began to send them out two by two, and gave them authority over the unclean spirits.(Mark 6:7)
The apostles returned to Jesus, and told him all that they had done and taught. (Mark 6:30)*
In General, both Paul and Luke on the other hand, use the word Aostle to refer to the whole group of believers or those who evangelize in foreign lands. Most of the references to the twelve apostles in the Gospels are simply “The twelve.” There are twenty three references to “The twelve” in the Gospels. Even then however, Luke acknowledges the twelve are Apostles.
And when it was day, he called his disciples, and chose from them twelve, whom he named apostles; (Luke 6:13)
We can parse the word Apostle all day long but there are clearly twelve people to whom Jesus revealed himself fully, and then those who learned from these.
-Tim-