I notice they have Vala shown front and center, but I'd much rather use electron with React so I don't have to learn a whole new language. Would they accept a submission in React?
Your app may be written in any language, but the front-end must be a native Gtk3 app. Web, Electron, Qt, Java, and other non-native app front-ends will be rejected during the review process. A game may be excepted from this requirement so long as it uses native window decorations and is generally usable on both loDPI and HiDPI displays.