Hi Norris, I can't comment on the temps, but supposedly July is the "best" time to climb Huascaran. I'll be there in July to do Pisco, Chopicalqui, Alpamayo and finally Huascaran so I certainly hope that's the case!! Suerte
Typical lows during the night at Garganta would be in the order of -10 Celcius.
Typical highs during the day high on the mountain are just below freezing, but the incredibly strong sun can make it feel much, much warmer. Conversely, as soon as the sun goes away, it suddenly feels much colder again.
And yes, July is the best time in the Cordillera Blanca, but it's not an exact science. Quite a few routes can already be in good shape by early June, and many can still be fine well into August. At Casa de Guías in Huaraz they'll know about current conditions.