GP4000S measured width:

on a typical 14mm inside / 19mm outside width rim ( rim is from 2010.)
25c is 25.8 mm tire width

On HED Ardennes plus wide rims, 20.6mm inside / 25.5 mm outside width rim:
23c is 26.7 mm tire width, and this works great on this rim.
25c is 29.0 mm tire width (and 25c GP 4-Seasons is the same width.)