When you receive an email from the list, the reply-to address will differ depending on whether the topic email was created by a user has Mailing List Mode selected or not.
For a user not running in Mail List Mode, the reply-to address is email@example.com
For a user running in Mail List Mode, the reply-to address will look like this
This form is a reply-to address generated by Discourse that links back to the topic. This address is used when Reply List is selected and will work with both types of emails.
In order to reply to the list with GMail, the reply settings need to be changed to Reply All