In season season 2 episode 10 "Sweet Dreams" Uther is discussing a peace treaty with another king whos daughter "Lady Vivian" Is his pride and joy and if any man touched her it would ruin the peace treaty. A third party king who would be poor if the war ended asked his servant to enchant Lady Vivian and Arthur to fall in love.
When they do Arthur says to Merlin to send flowers to his love not specifying Lady Vivian Merlin places flowers and a note which says "All the barriers that keep us apart cant beat the power of true love - Arthur" Gwen is overjoyed and tells Merlin she is having the best day and that sometimes when you think everything is lost something happens to give you hope. Gwen leaves a note in Arthurs room unsigned that tells him he can have a second chance and to meet her in her room. However she leaves it unsigned and Arthur goes to lady Vivians room leaving Gwen waiting there until her candle goes out and Gwen is in despair. However Merlin tells her the only way to break the enchantment is to give him a kiss from his true love gwen kisses him and Arthur is returned to normal and saves Camelot from War. Proving Arthur is Guineveres true love and Guinevere is Arthurs.
In season 3 episode 10 "The Queen of Hearts" Morgana has a dream of Guinevere being crowned Queen of Camelot and in pain and jealousy decides to reveal their relationship to the King. She encourages Arthur and Gwen to spend time tells Arthur and Guinevere to go out together. Merlin organises it and they go out for a picnic while this is happening Morgana asks Uther to go on a ride with her and she leads him to Arthur and Gwen when Uther sees his son in love with a servant girl he is furious.At first Uther laughs and says he also had a liking for servants when he was a prince but when Arthur continues to demand he is in love with Gwen Morgana hides a magical pouch full of herbs under Arthurs pillow making it seem like Gwen had enchanted him. Gwen almost dies but Merlin saved her. This only strengthened Arthur and Gwens relationship <3
Later in the season...In season 4 episode 9 "Lancelot du Lac" Arthur had asked Guinevere to marry him when Morgana found out she would do anything to keep a serving girl from her throne. Morgana uses necromancy to bring Lancelot back. then using magic brought back Gwen's feelings for him. Gwen and Lancelot are caught by Arthur who is furious and almost kills Lancelot but Gwen stops him, Gwen is thrown in jail. Crying, she throws off the enchanted bracelet from Morgana which caused her unnatural attraction to Lancelot. Afterwards to save Gwen from being put to death like his advisers suggest Arthur tells Gwen to leave camelot and never return. She asked where she would go but Arthur told her he didn't know. Arthur was so blinded by jealousy rage and heartbreak he sent her away. There was nothing else he could do. This was the lowest point for Gwen and Arthurs relationship.
Season 4 Episode 13 "The Sword in the Stone Part 2" Gwen and Arthur at long last marry. After seeing Tristan's lover die in his arms he realizes he cannot lose Gwen again. Arthur comes to Gwen as she is cleaning a room and asks her to marry him, as he never stopped loving her. She responds, "yes" with "all her heart".