I have not tried this myself, but as per this comment on GitHub from one of the Expo team members, I think there should also be an ios.googleServicesFile. I believe the app,json docs are out of date, since they do not mention this option. @wkozyra, can you confirm?
Anyone know for sure? I submitted an update to my app with it pointing to the .json file and still didn’t get any Firebase analytics for iOS to show up. Android works. Will try again pointing to the .plist file.