This article contains plot/ending details.
At first thought to be his own personal group of "Evil Exes" by Scott, Gideon's Ex-Girlfriends were considered "the ones that got away" by none other than Gideon himself. These girls are: Courtney, Riley, Nina, Sloane, Irene, Ginger, and Ramona. While Ramona was never added to his "collection," the other 6 are kept in a large hanging Cryogenic Freezer inside Chaos Theatre. Revealed in Volume 6, Gideon's master plan was to thaw them and date them at a time of his choosing, but his plan would take place only once Ramona was frozen amongst them. His plan was foiled when Scott and Ramona defeated him, and all 6 were liberated.
Their stories and opinions of Gideon are unknown, though assuming they've been frozen against their will after becoming his ex, they probably don't think too highly of him.
It is not known when Gideon started "collecting" exes, but Irene claims over the phone to have been frozen since 2003. Considering this is the fifth ex-girlfriend, and considering Gideon's history of not handling breakups very well, he might have been at the ex-napping game for years beforehand.