wxFormBuilder is an application that can literally speed up GUI development, being Python either C++ the programming language of choice.
To install it, first add the gentoo-zh ebuild repository:
layman -a gentoo-zh
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:
emerge --ask =dev-util/premake-3.7
You may now proceed and install dev-util/wxFormBuilder itself:
emerge --ask wxformbuilder