GPG offers a lot of options that are not supported by OpenPGP. GPG keys may work but only OpenPGP keys are supported. Yes you can use the same keys with webcrypt as with OnlyKey agent.
An easier way of troubleshooting might be to start with a known working configuration. Create protonmail ed25519 key and you would be able to follow the guide to load and use with webcrypt and OnlyKey agent - Import keys from Keybase, Protonmail, and Mailvelope/GPG | Docs