It is, mostly, a once blommer in my experience. Only a few roses will appear in the autumn, but it seems to be more of a 'mistake', as it happens with some pimpinellifolia or with City of York, than an actual second flush.

As for the hardiness, yes it is suprisingly hardy for a hybrid gigantea. I have seen it withstand quite well -15°C, when, for instance, Cl. Mlle Cécile Brunner died back to ground level. But then again, if it is Irene Bonnet, as suggested, it would seem to be reasonably hardy for a hybrid tea.
Although if such, it seems quite odd that it is a once-bloomer.