I absolutely wouldn't risk it, but it is your choice if you are comfortable with that tight a connection. Even in Switzerland, trains can be late and you have no leeway with that timing at all.

I agree with the other person replying. I was in Switzerland this past Fall and took trains every day all over the country. While the trains there are marvelous and 99.9% PROMPT, we did have one day when there was some malfunction in the part of the system where our train was travelling and we had a delay of more than an hour when we just sat there. This is extremely rare (it made the news that day in Switzerland) but it does happen. If it was anything other than your return flight that you were catching in Zurich, it would be worth risking, but that is tight for an international flight.

While Swiss trains are very reliable, it is not perfect. This is the case for any mode of transportation. I had one train from Geneve just suddenly canceled with 40 min notice. If the consequence is not manageable, I wouldn't do it. In case of city center to airport, if metro is not running, there may be a shuttle bus, or taxis and taxis, so there is no shortage of fall back options. In your case, your alternative may be taxi to Zweisimmen/Spiez (yikes$$$) then start your train trip from there. Your next regular train from Gstaad is one hour later.

IME of riding zillions of Swiss trains the bit about them operating always on split second timing is a fallacy - trains are often late - rarely by very much but often IME late - no better or no worse than other countries IME.

But my philosophy is that you should always be in your departure city if you have a non-refundable flight to catch, especially fairly early in the morning as you never know what may pop up - a rail line closure due to accident, avalanche (as has been the case in the rail line to Zermatt in the past few years at times) - strikes (rare here I think but you never know).