The rules on somatic components are clear:
A somatic component is a measured and precise movement of the hand. You must have at least one hand free to provide a somatic component.
A naga generally casts spells like a cleric or sorcerer, which should mean that they follow the same restrictions as those classes. Does this mean that a naga and creatures like it with actual spellcasting abilities cannot cast spells because they don't have hands?
The Monster Manual for D&D 3.5 in its glossary entry on Spells included this:
A spellcasting creature that lacks hands or arms can provide any somatic component a spell might require by moving its body. Such a creature also does need material components for its spells. The creature can cast the spell by either touching the required component (but not if the component is in another creature’s possession) or having the required component on its person. Sometimes spellcasting creatures utilize the Eschew Materials feat to avoid fussing with noncostly components. (315)
…Yet similar text is absent from the Bestiary for Pathfinder. So this became kind of a big deal, discussed at length in a 2010 Paizo messageboard thread. In that thread, creative director James Jacobs eventually says in this post that
For humanoids, somatic components include hand gestures. For things with other shaped bodies, somatic components include whatever gestures their body naturally makes, be that paw or leg movements, tail wagging, squirming bodies, or whatever.
…And the thread is shortly thereafter locked, the matter considered resolved by the Pathfinder staff.
