Just to clear things up – Ethos ( or any of the Sites from Library ) are not themes. Their simply a pre-built site that is imported, very similar to if you use a Duplicator / Export plugin to copy your site to a new domain name. So underneath they are all just GeneratePress 🙂
All of the well coded form plugins will work with GP.
This one recently came up in our FB community as being one of the favourites:
https://wordpress.org/plugins/fluentform/