You have this set in your apps: block:
apps:
transgui:
command: transgui
This won’t set the library paths and other desktop-related stuff for your app via the desktop-launch script that you added with the desktop-gtk2 part. Instead, change it to:
apps:
transgui:
command: transgui
command-chain:
- bin/desktop-launch
That should fix your missing libgdk-pixbuf