Here is an article you might find interesting:
https://www.bunnymeadow.co.uk/blog/2019/2/27/vitamin-d-deficiency-in-indoor-rabbits-and-guinea-pigs
A lot of the studies on which it is based - see footnotes - are about Rabbits and I think the truth is that the science for GPs is probably unclear, as they are not very well researched animals. However, what does seem to be clear is that Guinea Pigs need sunlight in order to get vitamin D.
I think (based on my subjective opinion only, to be clear!) its probably the case that most creatures that are used to living in daylight might suffer some mental health impacts of being without daylight at all for six months, which seems to be the impact if you put them inside now, as I would assume that it won't reliably get this warm again until late March. As mentioned above perhaps a SAD lamp and good exercise and other stimulation could help to mitigate this to some extent.
Have you tried putting thermometer in your hutch to see how warm it is overnight and during the day? Having this information might help to weigh up the costs and benefits of moving them in the garage, including whether you could realistically wait another month?
I know this is a very tricky one. We had originally planned to keep our guinea pigs outside in a hutch throughout the winter. In practice, it got too cold and we ended up with them in the house in somewhat temporary accommodation from November to March. After that, we got a shed (with a window) specially for them and put the hutch inside it, which is a better solution for us. Of course, it depends on your available space and resources.