Ref T10747. In the in-email ICS event card that Gmail shows, it has a "Who" field which reads "Unknown Organizer*" if the URI for the organizer isn't email-address-like.
Previously, we used a URI like https://phabricator.install.com/p/username, which I think is OK as far as RFC 5545 is concerned, but Gmail doesn't like it.
Instead, use PHID-USERemail@example.com, which doesn't go anywhere, but makes Gmail happy. Users don't normally see this URI anyway.