The 25-year-old singer - who briefly dated the model last year before rekindling their romance recently - is ''in love'' with the blonde beauty, but friends fear he is set for heartbreak as she only wants to ''have fun''.
A source said: ''Joe really wants to move forward to the next level -- being a serious boyfriend/girlfriend. But she says that she just got out of a relationship so she's not ready -- but he's in love with her.
''He sends her flowers and says how hot she is and talks about her all the time.
''He thinks she's gorgeous and wants to date her exclusively. She just wants to have fun and casually date. His friends think she's going to break his heart.''
Though the pair spend a lot of time together, the 20-year-old model is said to see their romance as just a ''rebound'' following the end of her relationship with Cody Simpson last month.
The source added to the New York Daily News newspaper's Confidenti@l column: ''They hang out and make out. They're dating, going out to dinner.
''They're cute and affectionate together. But it's more of a rebound in her mind.
''They dated a while back as well, but Gigi broke up with Joe before she started dating Cody.
''Joe always cared about her since he got dumped [by Blanda Eggenschwiler] and thinks she's just the most gorgeous girl.''
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