In general everyone say realtime is best for the performance of the application but is it good to have all the applications as realtime ??
There are some cases where polling might be better than real-time streaming. Essentially, it's when you have a massive event stream and the client cannot easily cope with this stream in real time. For example, you are pushing tons of events to a mobile device that dequeues the data more slowly than the producer. In such a case, thanks to polling, the client could ask for a new batch of data, process it quietly, than ask for another batch. Of course, all this makes sense if the data producer (the server) is able to resample the data flow so that at each request, it doesn't need to send all the same data it would send when streaming.
So, to go back to your specific question, both Gmail and Google Drive do not produce so much real-time data to need polling (I know this sounds counterintuitive!), and I would then say that real-time streaming would always be better than polling. But streaming is a bit more delicate than polling). You must monitor if the connection is healthy. It could be half-closed or half-opened and you need bidirectional heartbeats to make sure it's fully alive. In case of disconnection, you must be able to automatically reconnect and restore the state before the connection broke.