1. A person who is careless in either actions or remarks in a way that is often offensive and/or sarcastic but usually not too serious. The term is used playfully because a person being a Gumba, much like the characters of similar name in Super Mario Bros., is not to be taken as a serious threat, just playfully annoying.
Many Gumbas don't know how weak they are or else they would never behave in ways that encourage others to stomp on their heads.
2. An insult used when you want to seem more polite than you feel. i.e. "It's nicer than calling someone a retarted