You can follow one of two methods below:
Go to Gmail > Settings (gear icon)
Under the Accounts tab > In the Send mail as section, add the email address that needs to be created as alias.
Once the alias is verified, go to Freshsales - Click on your profile picture on top right corner > Connect Email > Remove the integration by clicking on the trash icon and reconnect Gmail once again.
Now, try composing an email and check if the alias addresses are appearing.