Gryffindors have never gone so long without winning
I don't know the meaning of that sentence. I searched "never do without doing". That means you should do something, but I can't get it.
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It is an idiom: to go without (something). Try searching for go without, not do without.
A week is a long time to go without eating.
I have never gone so long without eating.
How long can someone go without water?
Your sentence means, "Gryffindors have been losing for a long time: their longest time ever."