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Many cards on gatherer have some amount of Gatherer Rulings (for example, Wild Evocation has quite a few).
Are these rulings just a shorthand and reminders of the relevant bits of the rules (i.e. if you knew the complete rules of MTG off-by-heart and how to apply them perfectly, then you would be able to derive everything in the Gatherer Ruling)? Or do these rulings fundamentally change the rules of MTG (i.e. the card would function differently if the rulings were not there)?
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.
Typically they are just reminders or tips on how the interactions play out, but sometimes they are notices that the card works differently than how it is read. these are called "errata."
As an example, look at Impulse. Original printings said you shuffled your deck after putting cards on the bottom, but the errata removes that.
