Everyone I email uses Gmail. What good is this?

Less than we would like. The guarantee holds between DMCN inboxes. At the bridge it drops back to ordinary email with a signed authentication verdict attached, which is better than the same verdict being worked out and hidden from you, but it is a compatibility trade rather than the point.

It suits bounded groups where both ends can plausibly be inside: an organisation's internal mail, a professional community, a group of self-hosters. If most of the people you write to will never be on it, there are better things to use.