This is by design, specifically gmail’s design. In order to send email as an alias through a gmail account, you’ll have to register that alias. See https://support.google.com/mail/answer/22370?hl=en for instructions.
I create a number of forwarding inboxes for domains, for example, photos@mysite.fake forwards to myfakeemail+photos@gmail.com. Because mysite.fake is a GSuite account, I can register photos@mysite.fake without SMTP settings. The same is true for all gmail accounts. For other accounts, you’ll need to provide smtp settings.