This appears to be a common snag for those of us using a customised email address or specific domain, but wishing to use Google’s fantastic mail servers.
I feel that it’s safe to assume that if you’re reading this, you’ve set email up before but can’t understand what you’ve missed… …and the answer is nothing! It all boils down to the initial set-up when we selected “Gmail” because that’s what we’re using. But by choosing “Other” instead, we get the extra options we need to set the ‘from’ and ‘reply-to’ email addresses. So, all you need to do is, delete your current Gmail account on your iPhone, then start from scratch, paying attention to the screen shown below. 
I feel that it’s safe to assume that if you’re reading this, you’ve set email up before but can’t understand what you’ve missed… …and the answer is nothing! It all boils down to the initial set-up when we selected “Gmail” because that’s what we’re using. But by choosing “Other” instead, we get the extra options we need to set the ‘from’ and ‘reply-to’ email addresses. So, all you need to do is, delete your current Gmail account on your iPhone, then start from scratch, paying attention to the screen shown below.
It’s pretty much the same on the Mac too, but then how often do you need set email up if you’ve had the same email address for over ten years..?