Florian's comment explains that the "Sandbox" prefix will disappear in Production.

For the `senderDisplayName`, this [knowledge article][1] explains why this is a gmail issue and not a Salesforce issue.

This is a [security feature][2] set by Gmail which displays the full email address of the sender as the name doesn't match. I believe this should go away if you reply to the email in gmail and receive another. 

I would test in Outlook or another service to confirm it looks as you want as well. If replying to the email doesn't fix it in gmail, I would defer to the article for the two options below:


1. > As mentioned in the Google article following a best practice of using
> DKIM signing [Create a DKIM Key][3] in Salesforce combined with an SPF
> Sender Policy Framework ([Sender Policy Framework (SPF) and Salesforce
> SPF Records][4]) entry for Salesforce MTA IPs in the senders DNS should
> resolve the problem.
> 



2. > Another alternative could be to disable following email deliverability settings in Salesforce to remove the *bnc.salesforce.com from the
> sender domain.
>
> -Activate bounce management 
> 
> -Enable compliance with standard email
> security mechanisms



  [1]: https://help.salesforce.com/articleView?id=000329365&type=1&mode=1
  [2]: https://support.google.com/mail/answer/1311182?hl=en&ctx=mail
  [3]: https://help.salesforce.com/articleView?id=emailadmin_create_secure_dkim.htm&type=5
  [4]: https://help.salesforce.com/articleView?id=000315520&type=1&mode=1