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我爱你 = I love you, but 我的姑娘 isn't even Chinese at all!!! It's actually wrong: Someone tried to translate from English into Chinese but got the words wrong. 我的 = my; mine; belonging to me, 姑娘 = Miss, as in an unmarried young lady. "我的姑娘" is not valid Chinese grammar. 我的姑娘 DOES NOT mean "my girl" as someone said! I guess someone's trying to show off.

its not gramatically correct, more like someone took two separate phrases and stuck them together

我爱你 means i love you

我的姑娘 means my girl, or my little girl

so stuck together the way they are, it roughly means i love you my little girl

this means "I love you my girl" in Chinese.