As forato said, if they're not powered on temp doesn't really matter (within reason), but humidity does.
Basically, just don't turn them on until you let them slowly warm up. The temperature might differ but I wouldn't turn on any rig for hard use (e.g. mining) if it has been stored around 10-15°C.
Other than the benefit of heating the house with GPU rigs, the main reason why I don't run my rigs in my garage because while the whole garage would probably be around 20°C even if the outside temp is sub zero, if there's ever a storm or power outage, it would take literally minutes until water condensates on the cards.
Why would this be an issue? Condensation mainly happens when a cold item, such as the cards are brought into a warm humid environment. So if your cards are in garage and running, then the power goes out, the cards will cool off slower than the surrounding air, which is probably not that humid to begin with in the Wintertime. So say they are off for 12-24 hours and the cards are equal with the now cooler garage air temps, then the power comes back on, the cards will heat up before the surrounding air. In both cases the cards should be at or above the surrounding air temperature, never below.
I could see to definitely not store them in such an environment, but if they are running it shouldn't be an issue.
Winter is probably different, but all I can say from experience is that I have spent hours in the summer during storms with power outages in a thin metal plate garage (where the rigs were) soaking water off the inside of the roof where it was almost dripping in minutes from the condensation. Once the cards started running again (after a slow start, meaning idling for a while before starting mining again) it wasn't an issue of course as they made enough heat for water not to condensate on the walls inside.
I am running a significant number of cards this winter in my garage, so I am mainly double checking my logic as I do have concerns if all will go well.
I started running them in the garage this past summer because of too much heat in the house, and the worse experience I have had so far was when I left the back garage door open (for venting) and a storm hit with strong winds that practically had the rain coming in horizontally. It rained a bit on one of the rigs about 12' in from the door. Surprising, when my wife asked me if I left the door open because it was raining so hard, I went out and seen the rig was still running although there was visible water on it. I shut it off via the GUI and then unplugged it. I took everything apart and let it dry for a couple of days, as well as used a blow dryer to try and get the deep down moisture.
Anyway, I re-assembled it and it ran, but the GPU temperatures were very hot. Upon shutting it down and disassembling the heat-sink, I found the thermal compound had pretty much washed out. So I took all the affected GPUs apart and cleaned/reapplied thermal grease and tried it again. It is running for 6+ months now with no ill effects, but I do suspect it might end-of-life on me quicker than rigs that weren't rained on.
So I am not saying moisture or humidity is not a hazard, but I think modern electronics are a bit sturdier than we give them credit for.
My bigger concern with running out of the garage in the winter is a blast of super cold air when the garage door is opened on sub-zero days, but so far everything has been working out. In another thread a guy posted that lives in Alaska and claimed he ran his rigs in the garage will no issues, so I am banking on that experience as well.