Are you in the U.S.? Which state?

You're trying to establish a chain of custody here. The only person who remotely cares about the forensic integrity of the message is the judge (and the jury if it's a jury trial) and it seems like an argument you would like to make is "this email came from Google's servers so we didn't tamper with it" - but the judge won't take your word for it, you will need Google to substantiate that and they won't unless it's a workspace account.

If it's a workspace account, then your org already has a retention policy.

So again, what problem are you trying to solve and in which jurisdiction?