rm -rf ~/Library/Application\ Support/Google/Web\ Designer ~/Library/Preferences/com.google.WebDesigner.plist
defaults write com.google.WebDesigner.plist disable-gpu-rendering -bool true
6. Launch Google Web Designer.
I had the same problem on my Mac after updating to High Sierra. It was resolved after I completely removed and re-installed GWD:
1) Remove the Application
2) Remove ~/Library/Preferences/com.google.WebDesigner.plist
3) Restart the Mac
4) Download and re-install GWD