It’s been a while since I’ve taken a look at this so I’m confused as to what you mean by “it's impossible to 100% collect all items.“ Are items the term for the weapon cards?
There are multiple items in the entire Gallery: Cards, 3D, Enemies, Artworks, Store Items, etc.
The message was added because I noticed that there are a few people who are trying to 100% collect them all and end up frustrated because the amount of effort to do this is ridiculous.
Unfortunately, the Gallery is not designed to be collected at all. It's simply a place for people who want to look at certain items in detail.