The rotate images feature in one ZP Gallery is working as it should, but in another ZP Gallery, which exists on the same server (host) as the first, the rotate images feature is greyed out.
What I need to do to make it rotate images work?
You say that they are on the same server? Perhaps one of them is using GD and one is using Imagick. If that were the case, then perhaps one of the two libraries doesn't have the ability to rotate.
Beyond that, I have no clue why it would be functioning on one and not the other on the same host.
There is a plugin on the WEB site that provides image rotation when the GD library does not support it. Perhaps one installation has that plugin and the other does not.
At any case, You get the grayed out feature if there is no rotation support found. You might also check that both installs are running the same version of PHP. Many providers will allow a choice.
Check the overview page installation information. It will give you such information that you can compare between installations.