The 37-year-old became a dad for the first time when his girlfriend Courtney Mazza have birth to the little girl in September.
He said to UsMagazine.com: 'My girl thought we were having a boy the whole time.'
And when Gia made her entrance Lopez said he thought it was funny when he found out he had a daughter.
He said: 'I found it funny. I don't know why.'
Lopez added that Mazza 'is an awesome mum, a natural' and said she's already lost her baby weight.
He said: 'She's already back to fighting shape. I couldn't believe it. She's back to the weight she was before. She looks better than ever!'