There are many varieties of Gucci fakes, some are obviously fake, and some are copied quite well. If you buy Gucci online from a secondary source such as eBay, pay close attention to my tips in the 'Hunting Basics' section of this blog.
Over the years Gucci labels have changed. When Tom Ford was the head designer the labels and font were larger (as seen in the first example below). I think it was in 2005 that they changed the shirt labels from the rectangular shape to the longer and skinnier shape with smaller text (as seen in the fourth comparison example below).
Example (click to enlarge):

In the above comparison you can see the difference in the size of the lettering, both on the label and on the size tag. The difference is subtle, which is why it's important to look closely at the pictures when you're buying online. This is a Tom Ford-era tag, which is taller than the current tags.
Example (click to enlarge):

Example (click to enlarge):

The two examples above are rather obvious fakes. The first one is proportioned totally wrong, the Gucci text is wrong, and the size tag is wrong. In the second example the tag is gold, and there are no gold Gucci tags. Unless we're dealing with vintage Gucci, the label should be white text on a black label. The marked size (in this example 'M') should not be on the same label, it should be on a separate piece of material either sewn to the label or elsewhere on the shirt depending on when style shirt it is.

In the above example the fake tag is missing the 'Made in Italy' text.
Example (click to enlarge):

In the above example the fake tag is missing the 'Made in Italy' text.
Example (click to enlarge):
These examples are all of pretty decent fake labels. You have to click the pictures to enlarge them to see the variances in the text. The text in both of the above fakes is too bold, and the stitches don't form smooth rounded letters. Also note that the spacing between the 'Made in Italy' and the 'Gucci' text is greater in the authentic label.








