As you should know, gross also means total, so gross impression accounts for the total number of impressions made by a visitor on your website. Let's say Victor visits your website 3 times on the same day, on the first visit, he saw 4 ads before he left, on the second visit, he saw 5 ads, on the third visist he saw 3 ads - the gross impression will be 4 + 5 + 3 = 12.