A few months ago Google disable allowing less secured apps to be used to increase security. Here is what it says:
To help keep your account secure, from May 30, 2022, Google no longer supports the use of third-party apps or devices which ask you to sign in to your Google Account using only your username and password.
Is there a way to use git send-email with gmail now?
Yes, it's possible to use an app password for authentication. However, this requires that you've enabled 2FA (which you should do anyway). Once you've done that, you can go to https://security.google.com/settings/security/apppasswords to create the app password, as specified in the git send-email manual page.
I'm not aware of any way to do this without enabling 2FA if Google no longer allows the "Less secure apps" setting.