How do Gatherer Rulings interact with existing rules?
The rulings on Gatherer are simply applications of the Comprehensive Rules, taking what the card actually says and applying the rules. They don't change how cards work.

You could indeed deduce all rulings, but it's still incredibly helpful to provide them because the Comprehensive Rules are quite long and detailed and often abstract, so to most players it won't even be clear where to look for things, and of course if you're in the middle of a game you want a quick answer.

The rulings do also often clarify or explain the actual words on the card, that is, some of the deduction does also involve understanding English. For example, one of Wild Evocation's rulings is:


  If a player reveals a nonland card, that player must cast that card if it's possible to do so, even if he or she doesn't want to. ...


and that's because Wild Evocation says "the player casts it" not "the player may cast it." There's nothing in the comprehensive rules that's really going to explain that, it's just understanding what the words mean.