Yes, there is something that is equivalent to "GUI builder" in U++.
It is Layout Designer. To activate it, just start editing file with the .lay extension.
U++ does not generate the code - it is not needed. Instead, this .lay file is by some freaky process we tent to define as "multiincluding" converted to C++ templates that define layout of your dialogs (in other words, your GUI).
P.S.: BTW, instead of drag-drop widgets, right-click on layout to get menu.
P.P.S.: U++ is BSD licensed. The difference it that unlike GPL, you can use it for ANY purposes for fre..