How do I play without logging into GOG and play off line ?

Just launch the game directly through witcher3.exe in your game folder. You can also use offline installers. These create a shortcut directly to the game .exe. You can download the offline installers using your web browser. Just log to your GOG account point the mouse cursor to your nick on top of the screen, select Games, find Witcher 3, click on it and select "Download offline backup game installers". When you download offline installers, you don't need GOG Galaxy to install and run the game.

You may also find a non-Galaxy shortcut in the game directory (usually begins with "Launch [the game].lnk". It should point to the executable directly, with a nice icon too. Just copy-paste it on your Desktop.

I'm playing the game offline, no issues at all. Galaxy is just an added layer, you could even try launching through it and quitting Galaxy while playing and everything should still run fine (apart from Galaxy features of course, like play time and achievements).