Temp folder name issue.
With the given plist file extract:
<key>CFBundleName</key>
<string>GTK3-Demo</string>
<key>CFBundleExecutable</key>
<string>gtk3-demo</string>
GTK-Mac-Bundler runs with these values:
Temp location: /Users/me/Desktop/.GTK3-Demo.app
Final path: /Users/me/Desktop/GTK3-Demo.app
Correct but the following seems not:
Mkdir: /Users/me/Desktop/.gtk3-demo.app/Contents/Resources
That's not so critical as the latter is taken for renaming:
Rename temp to final app: /Users/me/Desktop/.gtk3-demo.app to /Users/me/Desktop/GTK3-Demo.app
If worthwhile, I might suggest a change.