Goblet of Fire

We first see Gabrielle when she is one of the hostages for the Four Champions in the Second Task of the Triwizard Tournament. Each of the Champions is given one hour to rescue that which is most dear to them from the bottom of the lake. Gabrielle is what is most precious to Fleur. Fleur fails in her attempt to reach Gabrielle, and is so relieved when Harry surfaces with her that she runs over and kisses him, and also Ron who had some incidental role in her return.

Gabrielle is present in the audience for the Third Task, though she plays very little part.

Half-Blood Prince

Gabrielle is mentioned in passing by Fleur as part of her planning for her wedding to Bill Weasley.

Deathly Hallows

Gabrielle is one of the bridesmaids at the wedding of Fleur Delacour and Bill Weasley. She arrives two days before the ceremony, with Monsieur and Madame (Apolline) Delacour, and follows Fleur around, prattling away at her in French and trying to help out.