When is a gimbal tripod head needed?

[utterdrong]

For long telephoto lenses, yes, but how long? And is it focal length, physical length, weight/weight distribution, or a combination of all of the above that actually matters?

I ask because I've been eyeing the Nikon 180-600mm with interest. It's a longer and heavier lens than I've ever wielded, but still considerably shorter and lighter than the behemoth lenses I used to see my peers mount on their gimbal heads. So I don't have a good grasp on whether or not I should be factoring that into the cost projection, or if a standard ball head will do the job.

Also, in my research, I've read about Acratech GXP and Flexshooter ball heads having some sort of special functionality for long lenses - are either worth considering as alternative options?

Thanks!