This must be a mistaken measurement. How would GMail take up 1GB of RAM? Maybe the user has some GMail addons that leak memory or load huge word dictionaries or something.
It's not false, I have this same problem right now. I have mail.google.com opened in a Safari tab. When the tab is open and In Activity Monitor, I see: Memory 1.2 GB, Real Memory 500 MB, VM Compressed 260 MB. When I switch away focus, Memory drops to 900 MB. The Gmail web app is a RAM hog in Safari.