I have ridden the GP4000S in 25mm for a long time. Mine are currently mounted to a DT Swiss RR1.1 Rim and they spec out at EXACTLY 25mm.
The rim is old school, (Trainer Mule now) and the dimensions are 14mm inner width, 19mm outer width. On that old Cervelo the problem wasnt width, but height. The diameter on such a narrow rim was larger, and as such the tire would rub against the (absurdly close) brake bridge on the back. I took care of it with a file, and all is good, though I’m not recommending that method.
I will absolutely go with tubeless 5000’s on my new road bike.