Tricky, depending on the volume you run into problems if you forward the mails. What works pretty nice and what I'm doing is let gmail fetch the mails via POP3

Tricky, depending on the volume you run into problems if you forward the mails. What works pretty nice and what I'm doing is let gmail fetch the mails via POP3 and configure it to send outgoing mail via the SMTP Server from the domain. No hassle about SPF/DKIM/DMARC and you can keep a copy from the mails on your server with the MTA and don't depend on Gmail. The from: is also from your domain and Gmail is clever enough to reply with the correct from: to mails that reached you that way. I'm using this with all my mail accounts and it's working fine.