I have looked at a lot of translation websites and lost of them have said that "gemein" means common but it also says that it means cruel.
Which one is it?
Actually it's both. The usage in the sense of "common" is a bit old-fashioned.
This is like the English adjective "mean". It's either common/general or nasty/cruel.
In the time of aristocratical German all non aritocratical people were called "das gemeine Volk" in the meaning of "the crowd of people". So "gemein" was something analog to "normal" or "common", and said from aritocrats with the meaning "nothing special".
Over time this changed to the worse. The meaning changed from "nothing special" to "something with bad behavior" and was now used from all people from all classes. Then the way was short to the meaning "cruel" in "Du bist gemein!" for "You are cruel", as answer when a child hits another, or someone steps intentionally on bugs...
I think a good example for the changing meaning is "gemeine Räuber" oder "gemeiner Dieb", (Räuber, Dieb: burglar) where the meaning is something stronger than "nothing special" but not in general "cruel". For example I could imagine a person in a crowd, that notice someone running away with their money and yelling "Du gemeiner Dieb!" but was not injured by this person. I am not sure, but maybe "gemein" could have in this case the meaning of "naughty".