Well if you'd like to know here's a real simple version.
Gems are aliens that project a physical body for themselves using energy. (Think Arnold Rimmer) These bodies all happen to look female, but being literally magic rocks, they actually don't have a gender.
At first /co hated it without this knowledge, I think because it had female characters in it or something? I genuinely don't know, but /co decided long before the show came out that it was TumblrSJWfemmnist poison.
After they found out about the genderless thing they got even more mad about pandering.
Amusingly Tumblr originally loved it as a haven for lesbianism and for having female characters that were cool.
Then they found out about the genderless thing and got really mad at everyone and each other over things like pronouns and that technically they aren't lesbians and such wank.