How can I use cbgreek with lualatex?

For small inserts in Greek, assuming you don't need correct hyphenation, the workaround of changing inputenc into luainputenc might do.

For more intensive usage of Greek, you should consider switching to fontspec. The CM-Unicode fonts incorporate the CB Greek fonts.

\documentclass{article}

\usepackage{fontspec}
\usepackage[main=british,greek.polutoniko]{babel}

\setmainfont{CMU Serif}

\begin{document}

\foreignlanguage{greek}{οὔ μοι τὰ Γύγεω τοῦ πολυχρύσου μέλει},
he said

\end{document}