I have a pair of these boots in a Gucci size of 7.5G. This is the size of Gucci shoes that I wear . . . normally. I have a few other pair of Gucci shoes to base this off of. In an Allen Edmunds shoe, which in my opinion are a shoe that fits true to size, I wear an 8.5 medium width.
These particular boots from Gucci seem to run even larger than their other shoes, you may want to keep this in mind when ordering and go down one more half size based on your normal Gucci shoe size you are accustomed to.
Secondly, the fit of the upper portion of this boot is very loose. I have several other pairs of non-lacing boots that are of the side zip up type (not Gucci brand). All of these other boots have the upper section (above the ankle) that fit much, much more closely to your lower leg (not tight). These boots provide a huge amount of room - too much room. I am not sure if they are designed to have your pants tucked into the top section of the boots or what . . . but there is just WAY too much space/material/girth in this upper section which makes these boots feel a little uncomfortable and even looser than their over sized nature makes them.
That being said, they are beautiful boots. I like the rugged sole, especially during winter in the midwest, when I am apt to wear them.
So. . . the boots are good boots . . . but the fit is very large . . . even compared to Gucci standards.