1. That's a pretty big question! Well, one thing they really try to downplay is the Incident, in which a botched spell summoned a god who transported an entire Magicademy ten years into the future. That was the specific reason Sundering was banned, even though there had been pressure growing against it for some time.

2. Gideon considers himself a 'generalist', as many wizards do -- which means he has a solid grounding in Discovery, Fantastics, Thaumaturgy, and Intimation; reasonable ability in Modification but poorer than a focused Modifier would be; and no knowledge of Sundering. This is generally what 'wizard' means to your average person, and for good reason.

3. Of course he likes ice cream!