Sonic and Gabrielle met in Spagonia before the events of Sonic Unleashed starts. Gabrielle (and Amos) were volunteering in Spagonia's L.Gaia temple and Sonic was passing by searching a place where to rest for the night after a long fight with Eggman. Gabrielle was really curious about knowing the Blue Blur because she would always read about him on the daily newspapers but she was kind of shy when it comes to speak with famous people. Luckily Jamala was with her at the time and pushed her to interact and help Sonic search for a place in the Temple where he could sleep for the night. From that day, they would start to get along when Sonic could visit her.
When the events of "Sonic Unleashed" started, both Sonic and Gabrielle then helped eachother out with the Dark Gaia's problem. Sonic would bring the humans and mobians who got affected by Dark Gaia and Gabrielle, alongside the other nuns and the Spagonian L.Gaia guardian, would exorcise them.
So, before Gabrielle turned into Surge, she knew Sonic for 2/3 years. She would develop a crush on Sonic, but because of her vows she would never confess it. Even Sonic was starting to really like this girl, but deep inside he's a shy dude too!