Did you choose the right output? Most packages only have a single output, but transmission has 2: "out" and "gui".
For example to install the gui output you would run: guix package -i transmission:gui The gui output is the one that might add your GNOME Menu entries as it comes with the GTK version of transmission. The normal "out" output is the CLI version only. This confused me the first time I used guix too, before I was aware of outputs.
