It’s not a free vs paid feature. You simply cannot send or reply with a non-profit domain that you don’t own. This feature has already been requested several times on the uservoice site
- https://protonmail.uservoice.com/forums/284483-proton-mail/suggestions/31264327-send-as-from-external-address-add-support-for-e
- https://protonmail.uservoice.com/forums/284483-proton-mail/suggestions/46229314-add-support-for-gmail-accounts-external-mail-acco
- https://protonmail.uservoice.com/forums/284483-proton-mail/suggestions/48450218-adding-imap-pop-and-smtp-support-for-other-email-p
- https://protonmail.uservoice.com/forums/284483-proton-mail/suggestions/32525668-smtp-out-to-allow-for-support-of-mail-forwarding-s
- https://protonmail.uservoice.com/forums/284483-proton-mail/suggestions/48817334-use-custom-email-address-via-smtp
- https://protonmail.uservoice.com/forums/284483-proton-mail/suggestions/47422739-abilitare-configurazione-smtp
and on the Reddit forum
- https://www.reddit.com/r/ProtonMail/comments/1ewc5i1/can_pm_send_as_without_changing_the_domain/
- https://www.reddit.com/r/ProtonMail/comments/1evg1ez/is_it_possible_to_send_mail_as_from_gmail_using/
- https://www.reddit.com/r/ProtonMail/comments/nw94m0/send_as_function_shows_as_sent_from_another/
- https://www.reddit.com/r/ProtonMail/comments/m23u81/feature_request_send_as_subaddress/
- https://www.reddit.com/r/ProtonMail/comments/gyl9r8/sending_as_the_direction_from_which_i_redirected/
- https://www.reddit.com/r/ProtonMail/comments/bmv41w/feature_request_a_good_reason_send_as_a/
- https://www.reddit.com/r/ProtonMail/comments/mrw81k/is_is_possible_to_use_pm_to_sendrecieve_as_an/
- https://www.reddit.com/r/ProtonMail/comments/7krs6w/send_mail_as_feature_for_protonmail_exists/
- https://www.reddit.com/r/ProtonMail/comments/8yzy04/is_it_possible_to_send_mail_as_in_protonmail/
auto-correct typo. Meant to say non-proton domain (like protonmail.com, proton.me, etc). You can forward the @gmail.com addresses to your Proton mailbox, but there is no way for your to send as those gmail.com addresses from your Proton mailbox in the same way you can send as non gmail.com addresses from your gmail mailbox. You can send as an address from a domain that you own and configure all the DNS records for Proton to handle.