My graphic card is : asus r7 370 what's the ideal temperature for the gpu ?

I would guess some where around 75 degrees Fahrenheit

Ideally, for any GPU, the temperature should be around 40-50°C (idle), and no more than 70°C when under load.

Of course, when the GPU is under load, it is very likely that the temperature will go over 70°C, and that's fine, as long as it doesn't go over 80-85°C, and the time the GPU is under load is relatively short.