WxFormBuilder

wxFormBuilder is an application that can literally speed up GUI development, being Python either C++ the programming language of your choice.

To install it, you need to first add the gentoo-zh overlay:

Then, since premake 3.7 is currently an ebuild prerequisite, though the core system is already providing premake 4, you have to directly specify it:

You may now proceed and install wxFormBuilder itself:

Official Documentation
See http://wxformbuilder.org