Seems like a frivolous practice. No - they're absolutely NOT required in California for photographic prints, or anywhere else I'm aware of. There are legal conditions to what can be sold as an actual painting or true lithograph, versus mass-produced alternatives. It would be easy enough to fake a "Certificate of Authenticity" for just about anything anyway. It happens all the time, for some kind of product or another. Even very very expensive galleries have been caught doing that.
But as an optional feature, such certificates might accompany limited edition portfolios, etc.
What individual galleries require as business policy is up to them. But if a living photographer is directly involved, that should be sufficient in its own right. Once things get marketing silly, I begin suspecting the ethics of the gallery itself.
Anyone with sufficient experience should be able to discern in person a skillfully printed real photographic print from a mass mechanical version or the work of a fraudster. Whether that is being done under the photographer's authorization or not, or that of his trust, might involve a bit of extra paperwork, as in a case like the AA Trust, where questionable works have brought on lawsuits. But in that case, the prints in question looked inferior anyway - a different past photographer of Yosemite, quite skilled for his era, but devoid of the same level of poetic sensitivity. I saw through that immediately, just by the nature of composition. Even in AA's early photographs, as technically deficient as many of them were, there was already a germ of that brilliant signature handling of light to follow. Just using the same kind of glass plates, or then film, around the same time, for the same type of subject matter, doesn't equate to being the same thing.
In a past life, I sometimes had to sleuth painting fraud using the copystand techniques of those days - infrared light, TechPan film etc. Once I discovered 19th C underpainting of something equivalents to "Dogs with Cards" beneath a faux Old Masters oil painting. I could spot the ruse anyway, from clear across the room instantly, due to its sheer inferiority of technique and composition. But let's imagine it was a well done fake; and in that case paying me a few hundred dollars for a simple evaluation made a lot of sense before spending thousands and waiting many months for the opinion of an expert.