The geographic area called Galatia (see map) is part of modern-day Turkey.  When the apostle Paul goes on his first missionary journey (with Barnabus), they start churches in various cities in this region (Perga, Antioch in Pisidia, Iconium, Lystra, Derbe, etc.) These churches are the recipients of Paul's letter to the Galatians.  As background, details of Paul's first missionary journey (to Galatia) are:

1. The Antioch church sponsors Paul and Barnabas mission to Cyprus and Asia Minor (see Acts 13:3,4)
2. They initially sail to the island of Cyprus (city of Salamis) with John Mark as a helper, then travel to the city of Paphos where Christianity is introduced to upper echelons of Roman socitey (Acts 13:6-12).
3. They sail to the city of Perga (north shore of the Mediterranean sea in southern modern-day Turkey). At this point, John Mark leaves Paul and Barnabus and returns to Jerusalem. Paul and Barnabus travel north to Anthich in Pisidia.
4. In Antioch of Pisidia, they receive favorable and unfavorable responses to preaching (Acts 13:42-45), and Jewish leaders convince the people to expel them, but the Gentiles rejoiced as Paul and Barnabus proclaim the gospel to them also (Acts 13:46-48) and Paul and Barnabus move on to the city of Iconium.
5. In Iconium, their preaching provokes division, and they are forced to flee when they learn of a plot to stone them (Acts 14:4-6) to the city of Lystra.
6. In Lystra, Paul heals a lame man, and the people believe Paul and Barnabus are pagan gods (Acts 14:8-12).  The Jews incite the people to stone Paul, dragging him out of the city beleiving Paul is dead. The next day Paul and Barnabus travel to the city of Derbe.
7. In the city of Derbe they preach the gospel and make many disciples. (Acts 14:21)
8. On their way back home (to Antioch in Syria), they make stops at the same cities they passed through, encouraging believers in each city, appointing elders, then make their way to Perga (on the coast of the Mediterranean) and sail back to Antioch (in Syria), Paul's home base.

The timeline of this first missionary trip is around the years A.D. 46-48.  

Then Paul visits Jerusalem (the Jerusalem Council in Acts 15), after which Paul writes his letter to the Galatians around A.D. 49 (to the individual churches he planted on his first missionary journey).  A main reason Paul writes his letter to the Galatian churches is that those churches were succumbing to and listening to the judiazing false teachers undermining the central NT doctrine of justification by faith (Rom 3:31), specifically teaching that Gentiles must first become Jewish proselytes and submit to all the Mosiac law before they could become Christians.  Paul wrote his letter to counter that teaching, to defend justification by faith alone, and warn those churches of dire consequences of abandoning that essential doctrine.

That (above) is the framework of Paul's letter to the Galatians.  After Paul wrote that letter, he then traveled on more missionary journeys (see below).  See maps of all of Paul's journeys here

Around AD 49-52, Paul goes on his 2nd missionary journey.   After initial disagreement between Paul and Barnabus about bringing along John Mark (who left early in the first missionary journey), Paul and Barnabus split -- with Barnabus taking John Mark via boat to Cyprus.   Then Paul takes Silas with him, departing Antioch and traveling north through Cilicia (city of Tarsus), through Galatia (cities of Derbe, Lystra, Iconium, Antioch in Pisidia).    Note: in Lystra, Paul meets Timothy - a young man who then travels with Paul and Silas, and to whom Paul writes later letters.    The three of them then continue on through Asia (see map) to the city of Troas, then sails across the Agean sea to the region called Macedonia and its cities (Neapolis, Philippi, Amphipolis, Apollonia, Thessalonica, Berea).  Then Paul sails to the region called Achaia (cities of Athens and Corinth) before he sails again to the region called Asia (City of Ephesus), and sails back to the region called Judea (modern day Isreal) to the city of Caesarea, then to Jerusalem, through Damascus, and back to Antioch.   During Paul's 2nd missionary journey he writes 1 Thessalonians and 2 Thessalonians from the city of Corinth.

Then around AD 53-57 Paul goes on this third missionary journey, where he writes 1 Corinthias (from Ephesus), and 2 Corinthians (from Macedonia), and Romans (from Corinth)

Then around AD 59-62 Paul goes to Rome and his first imprisonment (See Acts 27:1 - 28:31), writes Ephesians, Colossians, Philemon, and Philippians from Rome.

Then Paul travels to Macedonia and writes 1 Timothy, and writes Titus from Nicopolis (AD 62-64). Then Paul has his 2nd imprisonment in Rome (around AD 66-67) where he writes his last letter (2 Timothy) before Paul is martyred for his faith.