No—you can use a GorillaPod without a ball head, because the flexible legs can be bent to aim the camera.

That said, a ball head has real advantages. It makes framing much faster and more precise, especially for small composition changes, switching to portrait orientation, or aiming slightly up/down without having to re-bend the legs. This is also helpful when shooting multiple frames that need consistent alignment, such as panoramas.

A ball head is particularly useful when the GorillaPod is wrapped around a railing or other support, because the legs may no longer be easy to reposition. On a tabletop, changing the leg position to adjust the camera angle can also throw off the balance and take trial and error to fix.

So: you do not strictly need a ball head, but it improves speed, convenience, and fine control. If you're patient and only make occasional adjustments, you can do without one. If you use the GorillaPod often, a small ball head is likely worth it.