In the last year I have been showing Jupiter to the general public with a 100mm ED glass refractor with a magnification of 80-160, and when the GRS is on the disk, and it has been pointed out to them, 95% of them have been able to see it. This would be in the poor seeing we have been getting at the low altitude we have for Jupiter. So this gives a measure for people with no experience of looking though telescopes at all.
For more experienced observers I have little doubt a 75mm refractor would be enough, under typical conditions, with a magnification of 80.
I would say the GRS is easier to see now than it was in past decades. I think in the past a 100mm refractor or 150mm reflector would have been the absolute minimum, with good conditions.
I do have 80mm and 66mm refactors as well, so now the question has been posed, I’ll see if I and others can see the spot with those this year.