Dear community,
I am generally a fan of Bitwarden but so far, I have not found a way that automatically saves the used single-sign-on service (Google etc) that has been used to sign up for a specific service.
1Password automatically suggests to save it after using a SSO service.
Better, though, is to not use “Login with Google”. You have password manager. It can be used to nearly painlessly store unique passwords for each site. Using Login with Google means that if your Google account is compromised, so is your Reddit account.
There is a feature request thread here, where you can vote for this.
However, as already noted by @DenBesten, using federated logins controlled by third parties is generally not a good idea. The terms of service for all such services stipulate that the company can suspend, disable, or delete your account at any time for any reason (and they do regularly use this power, as you can confirm by running a Google search on the topic). This would cause you to immediately lose access to all accounts that used this service for logging in.