All my experiences with software-controlled fans have taught me is that it is better to avoid controlling fans in software. Sorry to say.
There used to be programmable, hardware fan controllers on the market, which would have allowed to do just what you ask, unfortunately, and AFAIK, these have gone the way of the dodo, as motherboard makers gobbled up that particular niche.
As a sidenote, what you want to control mainly is the airflow inside the case to stay as close as reasonably possible to ambient temperature: the efficiency of thermal exchange literally drops off a cliff when the temperature inside the case gets high. Ten degrees can make a lot of difference in terms of how fast the fans inside will have to spin, you can verify this yourself though (and it can be a fun experiment).
Personally, and broadly speaking, I don’t think 50 degrees on the GPU during benchmark or stress testing is anything to think about twice. IMO it already indicates that you have enough cooling, possibly more than enough.
Of course, all of this is assuming middle of the road conditions, if it hit 50 dC while operating the computer without the side panels and inside a tool shed in sub zero weather then it would be different. I’m just saying this, to get it out of the way since you never know who’s reading.