The centimetre number is a strap length
Gucci defines the belt size as the distance to the third hole. For a Double G style it runs from the outer edge of the buckle; for a standard buckle it runs from the beginning of the strap. A size 90 therefore does not equal a 90 cm body waist. The most reliable comparison is another belt that fits: lay it flat, find the hole you use and measure from the buckle edge to that point. This removes the difference between a waist measurement, a trouser label and a belt stamp.
Women's and men's ladders read differently
Gucci publishes a women's belt ladder from 65 cm to 120 cm and a men's ladder from 75 cm to 120 cm. The same 90 cm strap reads US 28 on the women's chart and US 32 on the men's, with different jeans columns. That is not an error; each chart maps the strap to a fit convention. Record whether a listing belongs to women's or men's sizing before using its equivalent sizes. The centimetre stamp remains the common value across both tables.
Canvas changes wear, not the size ladder
GG Supreme canvas is a coated material with leather trim. It stays flatter than soft leather and can mark first at the edge, especially where the keeper loop bends it. The printed surface may hide small scuffs better than smooth leather while showing edge wear more clearly. None of that changes the measured strap length. Ask for close photographs of the edge, the buckle, the keeper and the holes. A flat front image can show the monogram but not whether the strap is beginning to separate at the trim.
Width and buckle size are separate decisions
Gucci prints strap widths in inches. The common wide canvas strap is 1.5 in, about 3.8 cm; the metric shorthand calls it a 4 cm belt. The larger interlocking-G buckle dominates a narrow strap and reads more like a plate on a wide one. Compare the buckle width with the strap in a single photograph rather than judging the hardware from a cropped front view. Note the hardware finish too: gold-tone, palladium-tone and other finishes appear on different references and do not indicate a different size.
The four-part canvas-belt check
Ask for the full strap in frame, a tape from the buckle edge to the third hole, the number of holes and the width across the webbing. Add the interior stamp and the colour of the edge trim. If the belt is being compared with one already worn, match the same hole. For an on-waist fit, the placement changes the useful circumference and may point to a different row than a hip fit. That is a fit decision to measure, not a reason to guess one size down.