OK, there are two different "problems":
first: It is not a good idea to setup a mail server with a dyn IP, many providers check the RDNS => you won't have one. PÜlus if a mail server uses "old" DNS data, mails sent to your server may not arrive (IP Change).
second: If you use a smarthost (in your case gmail) the mail will be sent with this information as sender.
Best is to get a REAL Firewall and a static IP Adress.
For Excample: I get a static IP from my ISP. WAN-Data is entering through a FritzBox and forwarded to a Sophos UTM. Here I can define (internal) networks, forwarding rules,... And I also could use it as a smarthost. In this case, the UTM is like your Mailserver (plus SPAM Management, Virus checking,...).
Some Domain resellers offer free mailboxes. These could be used as Smarthosts, but it is an admin job I would not make (remember to change Passwords for Box and User,...)
Regards
Thomas