The list of sales on the Gilbert Literary Agency's site reads like a roll-call of dodgy and/or troubled publishers: Solstice (and an imprint thereof, Alpha Wolf Publishing, that seems never to have got off the ground), Muse It Up, Briona Glen, Grey Gate Media (formerly Briona Glen), Damnation, Karabeth (an imprint of Vamptasy, which has since been swallowed up by Crushing Hearts & Black Butterflies), Secret Cravings Publishing and Sarah Book Publishing.
Penrose Publishing is "by authors, for authors", Charles River Press is an offshoot or partner of Whiskey Creek Press, and Black Books Plus is a bookshop with a publishing arm, Enaz, that doesn't provide editing. Six titles were sold to World Castle Publishing, who require manuscripts to be "painstakingly edited and polished". One of the WCP books is by Christina Matson, who is also a member of the Gilbert Literary Agency's Submissions/Editorial team - they also offer proofreading and editing services - and describes herself on LinkedIn as a literary agent.
As has been mentioned, none of the publishers to whom the GLA has sold books require authors to have an agent.