Does Gothic 1 and 2 support 1080p resolution on Windows 8 64bit ?

And I mean proper 1080p not stretched image or anything like that.

The vanilla version does not. But there are unofficial Widescreen patches for both games.

Can you point me to a working widescreen patch for Windows 8 64 bit ?

See the following pages for details:
Gothic | Widescreen Gaming Forum
Gothic II | Widescreen Gaming Forum

The settings detailed there will make the games run at whatever resolution you set (the in-game Video settings will still show the previous resolution but the game isn't using it).

Note that for the Gothic 1 method you will need to make a shortcut directly to Gothic.exe; the shortcuts made by the GOG installer can't be used for this.

The default location for Gothic.exe is:

C:\Program Files (x86)\GOG.com\Gothic\system

If you changed the folder when installing Gothic then you'll find it in the "system" folder wherever you installed the game to.

Once changed the Gothic 1 shortcut target box's contents should look something like this:

"C:\Program Files (x86)\GOG.com\Gothic\system\Gothic.exe" -zRes:1920,1080,32

Finally, if Gothic/Gothic II shows a window border while running fullscreen follow these steps to fix it. The games will then display without it as intended.

If any of this wasn't clear just let me know. :)