Are you saying that all prints are manipulated or that there is no such thing as manipulation? Either way, I gather that you are not in favor of having a "digitally manipulated" print category for camera club competition, right?
The term "manipulated" in todays language usage, is IMHP, usually intended to indicate a negative idea.
To "manipulate" is to be devious in tricking someone to behave in a way that, fully informed and independant, they would not allow.
No matter how you try to apply "manipulate" otherwise, the underlying negativity and oprobrium carries with it, like a noxiuos odor to deningrate the subjected and/or object of that word.
The photographer, in capturing an image always ends up with a mere representation. For honest journalism or science or historical documentation, nothing most be added ever. Cropping without altering the facts of the matter may be O.K. depending on the rules of that documentation.
For your gallery, however, as I understand it, the purpose is to exhibit photographers work. sure locate sports, portraits, landscapes, pinup glamor or artistic nudes on separate walls if you like.
Composites, false colors or other changes are merely the obvious end of a continuous spectrum of artistic or editorial choices available to the photographer. Nothing more.
They either do it well and produce an emotional response or they don't.
A green giraffe or white cheeks or a car with wings may be trite or impactful art. That alone is for you to decide based on your pocket of rulers that measure value.
If, however, you wish to eshibit only pictures with certain criteria, inform the photographers offering their work.
In any case, let a picture speak for itself. I personally would avoid using "manipulated".