We already had a talk about this on /sug/ m8.
Jasper was born to fight the Crystal Gems in the war so she has always seen them as the enemy, and from what she knows they ended up taking everything from her and killing her diamond. She had to prove herself to earn her status and even though she is seen as a warhero she still always tries to prove herself and earn her right to be accepted on Homeworld because she thinks she doesn't deserve it for failing Pink Diamond and being from a failure of a planet. She didn't go out of her way to shatter Lapis or the corrupted gems did she? Her motivation isn't to shatter all gems that don't fit into the Homeworld system or obey the diamonds, it's to make up for her own personal failures and fear of being worthless, and seek justice against the Crystal Gems for wronging her diamond and colony. Her Homeworld ideologies are obviously brainwashed into her, they are the driving force behind her self loathing, and she's never had a chance to see the Crystal Gems as anything other than the bad guys. She values honor and justice, she's brave, loyal, and headstrong, she is always taking action to uphold her values and fights out of love for Pink Diamond. Her character has plenty of heroic and edgy anime protagonist qualities that fall in line with gryffindor even when you aren't looking at it from a Homeworld perspective.
Do you think preredemption Peridot or any other Homeworld gem has to be a slytherin just because they're brainwashed to believe in Homeworld ideologies too? I wouldn't be surprised because your unwillingness to accept anything other than "HURDUR slytherin=BAD GUYS/gryffindor=GOOD GUYS, Sugar is stupid!!" is a sign you're lacking some brain cells. Disregarding the hogwarts house classifications you should be able to comprehend subjective morality and depth in fictional characters. Rebecca obviously knows more about her OCs and puts more thought into questions regarding their values and motives than you.