I don't find that they run small, but personally the fit differs slightly between their various models (hi-top, low-top, leather, non-leather, etc.).
I currently own two hi-tops and both are leather. My usual sizes are US 8 or 8.5, 38.5 - 39.5 (depending on designer). For example, for Lanvin flats, 39.5 fit me best. For Nike running shoes, US 8 fit me best. For Isabel Marant boots (leather) 39 fit me best.
For GG, I own one pair in 38 and another in 39. They both fit, but I wear thin socks for 38 and can pretty much wear any kind of socks for 39. I've walked a few miles in my 38 and my feet were fine.
Also, note that they have a 'hidden heel' as part of the insole (both of my hi-tops have this and one low-top I used to own had it as well). Combined with the outer sole, it's close to an inch (or slightly more
I know the fit could be different between Mens and Womens (or how they feel on men vs women) but my husband has a pair in canvas, hi-top and they have slouched a bit (normal wear). However, the base/sole is sturdy and strong and after 3+ yrs of owning them they still look really good!