Gamut gut string gauges are approximate (≈) diameter. Meaning, that while a ≈0.60mm string is polished in the workshop to a diameter of 0.60mm, changes in ambient humidity, temperature, shipping, and storage can cause the natural gut string to expand or contract slightly.
Gimped gut strings and custom gauged equal tension strings are gauged with the equivalent-gauge (=) system. This means that the gauge listed, such as =1.50mm, indicates that the string is approximately equal in weight to a plain gut string of that diameter. Of course, because the wire is much heavier than gut, the string will be much thinner than a plain gut string.