Gouldians don't have down on them like other birds so feel the cold and draughts more, usually a nice sheltered and dry aviary is fine, i included a 40 watt light bulb in the night shelter of ours to allow them to feed of a night time and it was near a perch so they could snuggle up for warmth.
bred them and other exotic finches for 8 years, not too many problems.
i worked on the proviso that our night hours were longer than the tropics
so the birds had to burn all their energy on warmth not sustaining the body, so the light bulb was put in and seemed to work for me.
this is just my experience with them others may have other ways of keeping