A not-so-common distortion of the word "Condom", a distortion which originates in Russian slang. However, this word has nothing to do with condoms, for it simply describes a stupid person, or one who is inclined to act stupidly.
This word, just like Blatt, becomes increasingly popular in Hebrew slang, due to the last waves of Russian immigration to Israel in recent years.
A: "Man, what I did was damn stupid!"
B: "You're such a gandon..."