Fleur is seen as the perfect daughter/student and Gabby(who is Harry's age) feels left out, and not wanting to be in her older sister's shadow, rebels, dresses differently and does things to feel different, to not be measured by the same things as Fleur is, because she thinks she's not enough. Harry Potter has an undeserved bad boy fame by Fourth Year and Fleur is mad at him due to thinking he entered himself in the Tournament, so Gabby seeks him out to provoke Fleur, but ends up knowing the real him and falling in love
With the option of that Fleur also gets interested after knowing him better(perhaps through him trying to reconcile both sisters?) and now Gabby has all her insecurities thrown in her face because she's sure Harry would leave her for her older, more beautiful sister, but Harry is Harry.