Thank you for taking the time to report this issue and help to improve Ubuntu.
The plymouth package does not depend on GTK+. The plymouth-x11 package does, but this is an optional component and it's not a bug that this uses GTK+ as its toolkit. It happens to be installed by default on Kubuntu, but doesn't have to be; this is a choice for the Kubuntu developers to make, so reassigning to kubuntu-meta.
Thank you for taking the time to report this issue and help to improve Ubuntu.
The plymouth package does not depend on GTK+. The plymouth-x11 package does, but this is an optional component and it's not a bug that this uses GTK+ as its toolkit. It happens to be installed by default on Kubuntu, but doesn't have to be; this is a choice for the Kubuntu developers to make, so reassigning to kubuntu-meta.