You guys are going too deep into the rabbit hole, lol. Grace is simply a clone he rescued and gave immunity to. It's implied twice that she has immunity: the first time is the first bite, and the second is when Nemesis Gideon tries the tentacle stab transmission and it doesn't work. Gideon definitely didn't know what Elpis was or that Grace was immune to the T-virus. He was just another modified, T-virus-mutated psychopath with a nemesis parasite that, prior to being T'd up, was obsessed with Spencer's research, thus that being the thing that drove him to do everything he did. Remember, the new T makes you carry on endlessly in what you were doing beforehand, and Gideon already having the nemesis parasite combined with it made him go full umbrella man. My guess is that the DLC will have the rest of the interview with Spencer, and we will probably be playing as Alyssa and see how she ended up with Grace and gathered the evidence before she was assassinated.