That's difficult to say. There are indications that Paul thought his letters were not to be taken as "gospel":
But even if we or an angel from heaven should preach to you a gospel contrary to the one we preached to you, let him be accursed. As we have said before, so now I say again: If anyone is preaching to you a gospel contrary to the one you received, let him be accursed.—Galatians 1:8-9 (ESV)
However, the meaning of this passage isn't completely clear and in the same letter Paul claims:
Paul, an apostle—not from men nor through man, but through Jesus Christ and God the Father, who raised him from the dead—and all the brothers who are with me...—Galatians 1:1-2a (ESV)
Further the letter spends considerable words on the idea that Paul has equal authority to the leaders of the Jerusalem church. (See virtually all of Galatians 1 & 2.)
Meanwhile, Paul occasionally makes reference to teachings that are his opinion:
To the rest I say (I, not the Lord) that if any brother has a wife who is an unbeliever, and she consents to live with him, he should not divorce her.—1st Corinthians 7:12 (ESV)
He also talks in that letter several times of passing on things that were handed to him. For instance:
Now I commend you because you remember me in everything and maintain the traditions even as I delivered them to you.—1st Corinthians 11:2 (ESV)
We can extrapolate that Paul has three levels of communication:
- Traditions, whether oral or written down, that Paul holds in high esteem,
- Rules or commands that are his opinion, and
- Words that he did not designate in either category.
Since his words in category #3 are often strident, it would seem odd if Paul did not claim to have some authority from God. And if he has authority from God, what he writes might rise to the level of Scripture. On the other hand, Paul shifts without much fanfare from polite greetings to deep theological discussions to almost motherly advice to rich spiritual poems to attacks on his opponents to complex apologetic to friendly salutations. These happen all in the space of a single letter!
The evidence does not comport well with the popular assumption that when the writers of Scripture sat down to put God's Word to papyrus, they were ceased by the Holy Spirit and copied down exactly what the Lord dictated to them. At least in Paul's case, that doesn't seem the most appropriate model.
Paul's model of inspiration
Paul does at times refer to his own writings. An prime example is:
I do not write these things to make you ashamed, but to admonish you as my beloved children. For though you have countless guides in Christ, you do not have many fathers. For I became your father in Christ Jesus through the gospel. I urge you, then, be imitators of me.—1st Corinthians 4:14-16 (ESV)
Even more striking is this transition for talking about what Paul himself does in his ministry and the traditions about head coverings, the Lord's Supper, and spiritual gifts:
Be imitators of me, as I am of Christ.—1st Corinthians 11:1 (ESV)
By Paul's model inspiration tends closer to what the church now calls Apostolic Succession or, less anachronistically, Rabbinical Judaism. But where Saul the Pharisee traced his semikhah to Moses the lawgiver, Paul traced his apostolic authority to Jesus the law fulfiller (see Matthew 5:17). Paul sees himself as a vital link between the Christ and his spiritual children.
It would also seem that Paul claimed spiritual authority via the Holy Spirit:
But, as it is written,
“What no eye has seen, nor ear heard,
nor the heart of man imagined,
what God has prepared for those who love him”
—these things God has revealed to us through the Spirit. For the Spirit searches everything, even the depths of God.—1st Corinthians 2:9-10 (ESV)
Paul certainly intended his readers to take his letters as authoritative. He also might have anticipated that they would be incorporated into some collection of teachings as the rabbinic Jews collected the wisdom of the sages, prophets and teachers into collections as sacred writings. But we can't know if Paul anticipated the creation of the New Testament nor can we know if he expected his letters to be represented in it.