When Charlie first came home from the hospital after being born, Gabe hated her because he is now, "Just another middle kid". When Charlie puked on Teddy in Study Date, Gabe said, "You know what! She's starting to grow on me."
Since then, Gabe has started to like Charlie more and more. Although, he refuses to play with her in several episodes, and never wants to look after her.
However, this is just siblings being siblings. Deep down, Gabe really does care about his little sister. This is shown in an episode, where Gabe insults Charlie and she overhears. Gabe then wears baby clothes and sits in a pram to make her happy..