BDG is a french acronym that means : « bandeur de gadgi » or « bandeur de gadgo ».
It is used to talk about a person that’s always ready to f*ck with girls (gadgi) or boys (gadgo), or just a person that is easily horny because of them.
Your friend sees a very not sfw fanart of an anime character, and tweets that she’s his sexual slave and would suck his d*ck for hours : you can answer “bdg”.