For my primary email address, I will use Apple Mail. This program is set up to check my personal email address as well as my Gmail address. My personal email is used as default when sending email, and is digitally signed. Being digitally signed means that you can have assurance that the email came from me.
If I were to need to compose an email from my Gmail account, I will log into my Gmail service, and compose the email, or I will use Thunderbird (depending on most convenient). My Gmail account is not digitally signed, so you can only have my word that the email came from me.
If someone was to send me email to my Gmail account, I can see the email in Apple Mail, but my reply will use my personal email address. This is to keep things simple, and to the point.