Is there a good allergy medication similar to xyzal (levocetirizine) that doesn't cause weight gain?

Many. All of the antihistamines are similar, but different ones may work better for different people. Clarinex (desloratadine) is the only other second-generation antihistamine still available by prescription; cetirizine, fexofenadine and loratidine are all available over the counter. It is unusual for antihistamines themselves to cause weight gain, though - you should make sure that nothing else is going on.