wxWidgets provides for notebooks with multiple lines of tabs only on Windows platforms. Is there a simple way to provide similar functionality on GTK based platforms, or am I stuck with the only option being to create a customized notebook class that supports it?
As you say, you are stuck with the only option being to create a customized notebook class that supports it. GTK solves the "too many tabs" problem by providing left and right arrows with which you can page through the tabs.
Multiline tabs are a horrible idea anyway in my opinion. Take a look at these monstrosities.