Cassius and Gisela no longer trust each other after Gisela was revealed to be working with the Neverseen. Cassius cares more about his reputation than anything else, even his wife and son, and Gisela is extremely manipulative, a trait that was realized by her son, but apparently not her husband. The two are still married, but only in name as they are now living separately (Cassius at the Shores of Solace and Gisela at an unknown residence). They have no contact except for what little Keefe tells his father about his, Sophie's, and Fitz's communication with Lady Gisela, nor do they wish to have any. It appears that either both never loved the other, or that only Cassius loved Gisela, as the latter states in Nightfall that the only reason she chose him from her match lists was because she needed an Empath to assist her continuation of Lady Vespera's work. Gisela also calls herself and Cassius "incompatible."
Gisela and Cassius met at least fifteen years prior to Keeper, and it is stated that Gisela only chose Cassius from her match lists because she needed an Empath to help her with Vespera's research. It is revealed in Legacy that Cassius did love Gisela, which is why, even as an empath, he never knew she was evil and manipulative. They married and had one son, Keefe. Not much is known about their relationship until Neverseen. Keefe says at one point that Gisela makes Cassius multiple cups of slumberberry tea every night and that they each had their own areas of Candleshade where the other did not often go. They lived together in Candleshade until Everblaze, when Gisela was revealed to be part of the Neverseen. As of Flashback they are most likely still married, as Cassius would not want to file a match-fail (Elvin divorce) due to the scorn and reputation damage that come with it. Cassius is also working with the Black Swan against her, meaning that they are now enemies/rivals.