How to setup email forwarding for subdomains? e.g. [email protected] -> [email protected]

Question

I recently purchased a domain from Google domains, and am using email forwarding to forward emails to a specific Gmail address.

As of right now, emails are forwarded from an address under the format of [email protected]. Is it also possible to forward emails from an address under the format of [email protected]?

In DNS, I've created a new MX record subdomain.domain.com and set the value to match Google's default MX records. However, I am now stuck on how to forward emails from [email protected] to my Gmail account.

Whenever I use the normal email forwarding option given by Google domains and try to forward [email protected] to my Gmail address, it says "invalid alias".

Resources Used:

• Google Support (MX Records): Set up email forwarding with custom name servers
• Google Support: Forward your emails from email aliases
  ^{Didn't work for me. Only allows forwarding from [email protected]}
• Web Applications Question: How can I setup Google Apps Email for a subdomain?
  ^{This gave me the idea of mirroring Google's MX records, but this solution and others were for Google Workspace which didn't apply to me.}