I think that there is plenty air through the Gabber even with half the bars covered. Ours make 'summer nests' in the other corner of the tunnel bit which are more open if it is warmer. They never seem to suffer from the heat. With hamsters being so small they have a large surface area compared to their body volume so they lose heat quite quickly. Most hamsters I have heard to who have died of heat stroke have been in cars, sheds or in bright sunlight.
I am sorry to hear about your losses Basia. The mum must have been so dedicated to her litter she did not leave them even when getting too hot. Poor wee lamb. We did have an exceptionally hot time last summer and we are just not used to it really in the UK