Are there any accounts of former Pagans being supernaturally reached by God (or an angel) with the Gospel before any human missionary arrived?
This vary thing happened in the Book of Acts with the story of Cornelius the Centurion.
In the Book of Acts, Cornelius’ conversion happens because both Cornelius and Simon Peter receive visions from God. For Cornelius, God calls him and his Gentile household to follow the crucified and risen Christ. At the time, the fact that Cornelius was a Gentile (not Jewish) was a big deal. Being a Gentile was enough to prevent him from being accepted by the nascent Jesus Movement.
Cornelius was a centurion in the Cohors II Italica Civium Romanorum, mentioned as Cohors Italica in the Vulgate. He was stationed in Caesarea, the capital of Roman Iudaea province. He is depicted in the New Testament as a God-fearing man who always prayed and was full of good works and deeds of alms. Cornelius receives a vision in which an angel of God tells him that his prayers have been heard; he understands that he has been chosen for a higher alternative. The angel then instructs Cornelius to send the men of his household to Joppa, where they will find Simon Peter, who is residing with a tanner by the name of Simon (Acts 10:5ff).
The conversion of Cornelius comes after a separate vision given to Simon Peter himself (Acts 10:10–16). In the vision, Simon Peter sees all manner of beasts and fowl being lowered from Heaven in a sheet. A voice commands Simon Peter to eat. When he objects to eating those animals that are unclean according to Mosaic Law, the voice tells him not to call unclean that which God has cleansed.
When Cornelius' men arrive, Simon Peter understands that through this vision the Lord commanded the Apostle to preach the Word of God to the Gentiles. Peter accompanies Cornelius' men back to Caesarea. When Cornelius meets Simon Peter, he falls at Peter's feet. Simon Peter raises the centurion and the two men share their visions. Simon Peter tells of Jesus' ministry and the Resurrection; the Holy Spirit descends on everyone at the gathering. The Jews among the group are amazed that Cornelius and other uncircumcised should begin speaking in tongues, praising God. Thereupon Simon Peter commands that Cornelius and his followers, "kinsmen and near friends", be baptized. The controversial aspect of Gentile conversion is taken up later at the Council of Jerusalem (Acts 15). -Cornelius the Centurion
Some would consider Mary of Jesus of Ágreda of fitting the bill, while others may not. In any case I will post it as an answer.
Mystical bilocation and effect on missionaries
Between 1620 and 1623, Mary of Jesus reported that she was often "transported by the aid of the angels" to settlements of a people called Jumanos. The Jumano Indians of New Spain (what is today Texas and New Mexico) had long been requesting missionaries, possibly hoping for protection from the Apaches. Eventually a mission led by the Franciscan Friar Juan de Salas visited them in 1629.
The abbess reported further but less frequent visits afterwards, all while she physically remained in the monastery at Ágreda. They thus are considered bilocations, an event where a person is, or seems to be, in two places at the same time. Before sending the friars, Father Alonzo de Benavides, Custodian of New Mexico, asked the natives why they were so eager to be baptized. They said they had been visited by a Lady in Blue who had told them to ask the fathers for help, pointing to a painting of a nun in a blue habit and saying she was dressed like that but was a beautiful young girl. The Jumanos visiting Isleta indicated that the Lady in Blue had visited them in the area now known as the Salinas National Monument, south of modern-day Mountainair, New Mexico, about 65 miles (104.6 km) south of Albuquerque. At the same time, Fray Esteban de Perea brought Benavides an inquiry from Sor María's confessor in Spain asking whether there was any evidence that she had visited the Jumanos.
As reports of Mary's mystical excursions to the New World proliferated, the Inquisition took notice, although she was not proceeded against with severity, perhaps because of her long written relationship with the Spanish king.
Accounts of Mary's mystical apparitions in the American Southwest, as well as inspiring passages in Mystical City of God, so stirred 17th and 18th century missionaries that they credited her in their own life's work, making her an integral part of the colonial history of the United States. - Mary of Jesus of Ágreda
The following articles may be of interest in this subject matter: