A gwyhyr is a splendid gnomish sword, regarded as the best in the world. The gnomes had ceased production of these weapons more than two hundred years before the events of the saga. Gwyhyrs were forged with "dark iron" which made the blades light and sharp as a razor. They were also characterized by their undulating edge and hide covered hilts.
Gwyhyr appears in The Witcher 2 as a rare steel sword. It is the best steel sword available in Act 2. Geralt obtains the sword by completing the Baltimore's Nightmare quest in Chapter 2 if he sides with Iorveth. To obtain it, he must keep Master Baltimore's notes from Thorak (i.e. he cannot show him the notes!). this ultimately leads to the witcher being forced to kill Thorak after which he returns to Alderman Cecil and is rewarded with the sword.
Alternatively, showing Thorak the notes but rejecting his offer to let him search the workshop first will turn Thorak and his team hostile. This has the same result as not showing Baltimore's notes to him in the first place.