It is very similar to my place in summer.tempt in my greenhouse is now around 45-50° when mid day and 25-30 in night day ,
I cover my greenhouse by shade cloth in summer to keep room temperature within 50 'C.
If you could keep it within 50 'C, you are not necessary any counter measure.
If the Gymnocalycium is health enough, it should be in flower in summer.Btw astrophytum and some mammillaria flowering well in here, the problem is in some gymnocalycium species
A health Gymno forms new spines in winter. Do it form new spines in winter?
Frank