I'm not familiar with gentoo's packaging system, so I can't tell you what command should you use, but if you find a way to install it properly, you can help the project by writing a segment to the readme about the installing of dependencies in gentoo
Lightly half works for me on Plasma 6 following instructions on the qt6 version on GitHub. The window decorations and Lightly-related aspects of the plasma theme work, but I can't apply the application style.