Miss Geymonat posted on her facebook page: "In an attempt to calm things down, I started making jokes. I thought this might make them go away.
Chris even pretended she was sick, but they kept on harassing us, throwing us coins and becoming more enthusiastic about it.
"The next thing I know is that Chris is in the middle of the bus fighting with them. On an impulse, I went over there only to find her face bleeding and three of them beating her up.
"The next thing I know is I'm being punched. I got dizzy at the sight of my blood and fell back. I don't remember whether or not I lost consciousness. Suddenly the bus had stopped, the police were there and I was bleeding all over."
Miss Geymonat added that the gang of at least four men might have broken her nose during the ordeal, and stole a phone and bag from them before fleeing.
Both women were taken to hospital for treatment to facial injuries.
Miss Geymonat said one of the men spoke Spanish and the others had British accents.
London mayor Sadiq Khan described the attack as disgusting and misogynistic.
Siwan Hayward, director of compliance, policing and on-street services at Transport for London, described the assault as "sickening" and "utterly unacceptable", adding that "homophobic behaviour and abuse is a hate crime and won't be tolerated on our network".
Police are appealing for witnesses for the attack which happened in West Hampstead.
