Assuming that Gabrielle was invited, she of course would have gladly been there with a few baked goods. Maybe even something that Molly taught her to make as to show her appreciation! She would have wished Molly a happy birthday and offer her a hug.
Likewise, she would have also offered any other Weasley's & Potters a like a hug as well! If there was someone that didn't know who she was, she'd happily introduce herself as Fleur's younger sister or as Victoire, Dominique and Louis' aunt!