I have read the Arthur trilogy, and while i don't think it's absolutely necessary to have read them before this book, you will most certainly get more from Gatty's Tale if you have. Even if it is just for the small references to The Seeing Stone or At the Crossing Places which brought a smile to my face remembering reading them.
Gatty is the most wonderful heroine, she's innocent but strong, she's ignorant but sharp and bright and really very funny. Not many books bring a tear to this English student's eyes, but Gatty's Tale brought on more than a few! The story is beautifully told, and the reader becomes so attached to Gatty and her fellow pilgrims as they trek to Jerusalem. I felt as if i'd been on the journey with them, and the simple sentances and imagery is magical and effective. By the end, i didn't want to say goodbye to Gatty and the loyal Snout, not to mention all the characters at Caldicot.
This is one of my favourite books, a beautiful and accurate portrayal of medieval life but to get the most out of it, read the Arthur trilogy first, which is just as good, for only then can you reminisce wistfully with Gatty about Arthur and Pip, which when read originally, you wouldn't expect to find that image where it would eventually stay for centuries and more.