The story of Hosea marrying Gomer is primarily found in Hosea chapter 1. In this chapter, God commands Hosea to take Gomer, described as a wife of whoredoms, as his wife. This marriage serves as a symbolic representation of the relationship between God and Israel, illustrating themes of unfaithfulness and redemption.
For details about their relationship and its implications, you can also look at chapters 2 and 3, where the themes of restoration and God's enduring love are further explored.