The singer, 27, asked the actress and model, 24, to marry him earlier this week, according to the MailOnline.
Gabriella, whose real name is Gabriella Zanna Vanessa Anstruther-Gough-Calthorpe, is the sister of Isabella Calthorpe, who is married to Richard Branson's son, Sam.
CLICK ON PHOTOS TO ENLARGE
She is also the half sister and best friend of Prince Harry's girlfriend Cressida Bonas.
Gabriella has starred in films including The Three Musketeers, St Trinian's 2 and is due to appear in Carrie alongside Chloe Moretz and Julianne Moore.
Although acting is her main ambition, she made a foray into the fashion world when she starred in the Burberry autumn/winter 2012 campaign alongside Roo Panes.
Boyfriend Alan is the lead singer of the band Pale, who have supported Adele, Florence Welch and The Vaccines on tour. The musician also has a fashion background – he had a stint working for Roland Mouret before moving to Milan to further study fashion designer. A year in, however, he quit to concentrate on his music career.