Monteiro was born in São Mateus, Espírito Santo, Brazil. She started her modeling career at the age of seventeen and has travelled to Italy, Germany, France,Greece, Hong Kong, Singapore, Thailand and Philippines. She came to Mumbai, India in 2008. She began her acting career in Bollywood, portraying the role of the old-fashioned Punjabi girl, Harleen Kaur, in Imtiaz Ali's Love Aaj Kal which also stars Saif Ali Khan and Deepika Padukone. She was purposely kept under wraps during the film's promotion.Following the film's release, her identity was revealed, and it was also revealed that she had approached Ali through designerAnaita Shroff Adajania to audition for the role of Jo, Saif Ali Khan's Caucasian girlfriend in the second-half of the film, but, on his wife's suggestion Ali took her for Harleen's part, especially since he had not been able to cast for that role despite auditioning girls from all over India. She was featured on the covers of Indian edition of Harpers Bazaar, ELLE, Marie Clarie and Femina.

In addition to acting, Giselli is the brand ambassador of Wills Lifestyle,Movil Mobiles and PC Jewellers in India. She has walked the ramp for Rohit Bal, Ritu Kumar and Ranna Gill.

On 23 May 2010 Giselli appeared on the Fantástico broadcast by Rede Globo in her home country, Brazil speaking about Mumbai.

She next appeared in Always Kabhi Kabhi, a film under the Red Chillies Entertainment banner that released on 17 June 2011.

Lately she appeared on Westside, the chain of Fashion retail stores' new brand campaign Advertisement starting September 2011.

In early 2012 she became the face of Jhoomar, a fashion boutique based in Kolkata, India.