There are no easy ways to get scrolling bars on the display windows. You’ll have to program it by yourself (and I can already tell you, that will be probably harder than what you think).
As I just said above : there are a lot of limitations with the G’MIC display windows, just don’t expect you can re-create an entire graphical user interface with the G’MIC display capabilities. In theory, you can do it, but it would be cumbersome. I must confess I wouldn’t risk it myself.
If you need a real customized graphical interface for a GIMP plug-in, don’t use G’MIC, really. Do your plug-in with Qt/GTK/Whatever GUI Lib, and use C++ code, but don’t try doing this in G’MIC.
It is definitely not the right tool to do that.