Romans 3:7 needs to be understood in the context of an earlier verse in the same chapter:

4 God must be true, though every human being is a liar.

Thus, Paul admits that he - like everyone else - sometime lies. In Galatians, however, he is speaking about a specific truth claim: namely his account of his early history as a Christian, especially that he received his version of the gospel through revelation and that it was approved by the pillars of the Jerusalem church:

that the gospel preached by me is not of human origin. 12 For I did not receive it from a human being, nor was I taught it, but it came through a revelation of Jesus Christ.[j] 13 [k]For you heard of my former way of life in Judaism, how I persecuted the church of God beyond measure and tried to destroy it, 14 and progressed in Judaism beyond many of my contemporaries among my race, since I was even more a zealot for my ancestral traditions. 15 But when [God], who from my mother’s womb had set me apart and called me through his grace, was pleased 16 to reveal his Son to me, so that I might proclaim him to the Gentiles, I did not immediately consult flesh and blood,[l] 17 nor did I go up to Jerusalem to those who were apostles before me; rather, I went into Arabia[m] and then returned to Damascus.

  18 [n]Then after three years[o] I went up to Jerusalem to confer with Cephas and remained with him for fifteen days. 19 But I did not see any other of the apostles, only James the brother of the Lord.

Conclusion: Paul admits that he is a liar, as are all human beings. (Ps. 116:11) However, with regard to the his narrative of his reception of the gospel and his early history as a Christian in Galatians 1, Paul affirms that he is telling the truth

Romans 3:7 needs to be understood in the context of an earlier verse in the same chapter:

4 God must be true, though every human being is a liar.

Thus, Paul admits that he - like everyone else - sometimes lies. This does not mean he always lies. In Galatians, he makes a specific truth claim. He refers to his account of his early history as a Christian, especially that he received his version of the gospel through revelation, and that it was later approved by the pillars of the Jerusalem church:

that the gospel preached by me is not of human origin. 12 For I did not receive it from a human being, nor was I taught it, but it came through a revelation of Jesus Christ. 13 For you heard of my former way of life in Judaism, how I persecuted the church of God beyond measure and tried to destroy it, 14 and progressed in Judaism beyond many of my contemporaries among my race, since I was even more a zealot for my ancestral traditions. 15 But when [God], who from my mother’s womb had set me apart and called me through his grace, was pleased 16 to reveal his Son to me, so that I might proclaim him to the Gentiles, I did not immediately consult flesh and blood, 17 nor did I go up to Jerusalem to those who were apostles before me; rather, I went into Arabia and then returned to Damascus.> 18 Then after three years I went up to Jerusalem to confer with Cephas and remained with him for fifteen days. 19 But I did not see any other of the apostles, only James the brother of the Lord.

Conclusion: In Romans, Paul admits that he is a liar, as are all human beings. (Ps. 116:11) However, with regard to his personal history in Galatians 1, Paul affirms that he is telling the truth