In the Legends books, Arthur took one look at her and decided she was going to be his bride, with Merlin wasn't happy about at all. But I think her dad had a match in mind long before Arthur had the idea.
This is a link to Tennyson's poem, Gwenievere...my, my, my what a long read.
This is a modern day story, http://www.urbandruid.net/water/gwen.htm
This gives a brief history of when Arthur met Gwenevere. King Laodegan, of Carmalide, presented his daughter to the King, having her serve him the welcoming feast, as well as bath him (all the ladies of the court bathed the warriors) as they stopped on their way back from battle. Meanwhile, neglecting Merlin's prediction of her latter infidelity and outcome of , the two were united as one.
The last sentence in this little bit says that they had several sons and daughters together, and Arthur had several illegitimate children by Morgan Le Fay, Morguase, and many others. I just don't understand why there isn't more mention of thier children, it seems Mordred is the only one that anyone pays attention to. Of course...he's the bane of her and Arthur. According to the following link, her sister married Mordred and the two sisters had a fight, which led to Mordred "snitching" on her and Lancelot and basically it was the sister's fight that caused the Battle of Baddon.
According to this link she had a sister, the one above says she was an only child ????
The last paragraph of this is very interesting, referring to a triad and since the Empress is 3, adds a bit more spice to this card and her legend.
Giraldus Cambrensis says the cross claimed Guinevere as Arthur's "second wife". This appears to echo the story of the False Guinevere of French Romance: an identical half-sister of the Queen fathered on the same night who persuaded Arthur that she was his true wife. For two and a half years, the King was separated from the real Guinevere until the deception was uncovered. There is also an ancient Triad of the Island of Britain which records Arthur's "Three Chief Queens": Gwenhwyfar daughter of Cywryd, Gwenhwyfar daughter of Gwythyr ap Greidiol and Gwenhwyfar daughter of Ogrfan Gawr. This may further indicate the confusion over the lady's parentage as already alluded to. Alternatively, the three Guineveres could show a common Triple-Goddess motif at the root of many later Celtic characters.
Well, that stirred up the pot...but I think all I did was just confuse myself and didn't add anything really to the understanding of this card. But was a fun way to spend the evening and find lots and lots of Arthurian links and source material.