When a person (mostly found in girls) smiles & you can see an excessive amount of gum above their teeth. It can make a 10 star girl go to a 3 star girl, since one of the most important things in a person is their smile. People also use the "tooth to gum ratio" to decide whether or not there's too much gum. If you are unsure how to properly figure it out, look at the smile, then say to yourself "Is there an excessive amount of gum above her teeth?" The more gum, the uglier the girl is.
Alix: Bro, why are you dating Erin, she has a terrible gum smile!
Danny: Yeah I know man, but she gives good head
Alix: Screw the head! you don't want it coming from a mouth like that! Our dog buddy has a better mouth than that!