Is it possible to have 4:3 ratio in Larry 1-6? I can't seem to get it to do that. If I run them through a standalone ScummVM installation, the aspect works.

Any way to fix this with the GOG-versions?

Just started up 1 & 2. It works by default for me? Do you have the latest GOG installer?

I just re-downloaded, and I still get stretched display... :-(

You are correct - they configured the game wrong (sorry GoG).
Open the scummvm.ini in the Larry Folder (each game has its seperate folder) using notepad++.
Change the line aspect ratio = false to aspect ratio = true. No more stretched screen.

Original GoG config:
http://markoh01.lima-city.de/Larry%20falsch.JPG

Changed GoG config:
http://markoh01.lima-city.de/Larry%20korrekt.JPG