One day while Odin was away, the Vanir god Freyr sat on his throne, Hlithskjalf, from which he could look out upon the whole of the 9 worlds. As he looked upon the land of the giants, Jotunheim, he noticed a beautiful house owned by the sea giant Gymir into which a lovely young giantess entered.
Freyr became sad obsessing about the young giantess, whose name was Gerd, but he wouldn't tell anyone what he was brooding about; perhaps because he didn't want to admit that he had been sitting on the forbidden throne; perhaps because he knew love between giants and Aesirs was taboo. Since Freyr wouldn't eat or drink, his family grew worried, but were afraid to talk with him. In time, his father Njord summoned Freyr's servant Skirnir to find out what was going on.
Skirnir was able to extract the information from his master. In return, Freyr extracted a promise from Skirnir to woo Gymir's daughter Gerd for him, and gave him a horse that would go through the magic ring of fire surrounding Gymir's home and special sword that fights giants on its own.
After a minimal number of obstacles, Gerd gave Skirnir an audience. Skirnir asked her to say she loved Freyr in exchange for precious gifts. She refused, saying she had enough gold already. She added that she could never love a Vanir.
Skirnir turned to threats. He carved runes on a stick and told Gerd he would send her to the frost ogre' realm where she would pine for both food and a man's love. Gerd conceded. She said she would meet with Freyr in 9 days.
The servant returned to tell Freyr the good news. Freyr's response was impatience, and so the story ends.
The story of Freyr and Gerd (or Gerda) is told in Skirnismal (Skirnir's Lay), from the poetic Edda, and in a prose version in Gylfaginning (Deception of the Gylfi) in the Edda by Snorri Sturluson.
Source: "The Withdrawal of the Fertility God," Annelise Talbot Folklore, Vol. 93, No. 1. (1982), pp. 31-46.