Signed iOS app "can't be opened because it is from an unidentified developer" I am a registered iOS developer. I generated an IPA file using a signing identity that qualifies for App Store submission.

I sent my client this IPA file but macOS won't open it. It claims that this file comes from an unidentified developer. While there is a workaround by option-clicking the file, I want to know how a client can set Gatekeeper to trust all app files signed by a given identity.

It is likly that the client is running Mountain Lion or Sierra.
You've signed the IPA with an App Store distribution certificate, then not distributed through the App Store, so the certificate is invalid in the eyes of Gatekeeper and the file will be blocked. Use ad-hoc distribution if you wish to send the IPA file manually. Either way, macOS won't run IPAs as they're iOS app bundles.