Set https://github.com/Zebreus/install-neovim as the Dotfiles repo in your gitpod.io/preferences.
This repo contains a small script which downloads the neovim appimage and adds it to your $PATH
If you already have another repository for your dotfiles, you can probably just copy the content of install.sh somewhere into there.
If you need a specific neovim version, you can select it by specifying the NEOVIM_VERSION environment variable in gitpod. If the var is not set it defaults the the latest version. All neovim tags are neovim tags valid values (Like v0.8.0).