Can a turntable be stored in a garage?

Is it a Technics SL1700?

I wouldn't store anything like that in an unheated space.

Even if the plastic bags are airtight, the high and low temperatures can shorten the life of some electrical components. Also, a turntable probably has some lubricant which could vaporize and create a mess if the temperature gets hot.

I wouldn’t keep it in a garage for the above reasons. If you must, then buy some large desiccants and place them in the plastic bags to absorb moisture. Plastic bags are used to place the components in. Don't wrap the boxes with plastic.

Hi,
turntable: it can be safely stored, pack it as originaly shipped in all protective meterial, include some silicon gel bags, leave some oil in the bearing and seal it (moisture cannot enter there) apply a very thin film of light oil on any metal (ferrous) part (if any). Carton box may suffer if humidity is high but can be tightly wrapped to protect it even further.
Electronics and speakers: same as above but without oiling anything.
You can cover all inputs and outputs if not plated.
I have kept components in humid areas for over 10 years with no issues.