The gutter knights were the followers of Trystane Truefyre in King's Landing during the Dance of the Dragons. They were so-named because they were all knighted by Ser Perkin the Flea, even sellswords and thieves. These knights numbered in the hundreds.
During the Dance of the Dragons, violent riots occurred in the capital. Ser Perkin the Flea's gutter knights fought Ser Torrhen Manderly's northmen at Fishmonger's Square. The gutter knights managed to kill a quarter of Manderly's men, as the latter fought their way back to the Red Keep.
At the end of 130 AC, Ser Perkin and his gutter knights agreed to turn their cloaks and abandon Trystane's cause in exchange of pardons from King Aegon II Targaryen. Subsequently, they helped Lord Borros Baratheon to regain control of the city for King Aegon.