My experience with Barney's is that sizes listed on the site have been converted to the corresponding US shoe size and fortunately that was the case here. I'm a 10 so I ordered the 10 off the internet. The Barney's bar code stuck on the box has them listed as a 10 while the printing inside the shoe and on the G&G box mark them as a 9.5E. So, again, the 9.5 GG's are listed as 10 on the site.
As for different lasts requiring you to size up or down I have no clue as these are my first pair of G&G. My pair feel fairly true to size. They are a touch long but I'm really a small 10 and it's not an issue were I would consider sizing down. Hope that helps.
Barneys lists both the US and EU size for G&G.