I ran into the same experience about a month ago. Recently (2023) Gmail on Safari generates an error message from Google. Following the error message link to Google doesn't have any useful information. Running Gmail from Chrome or Firefox is not affected, so I don't think it's a corrupted mail box or index file.. Then, just this week, my iPhone started pausing a long time before downloading new Gmail. It's been spotty, though. If anything, I may need to rebuild the Gmail index, which isn't an option.