Can guinea pigs sleep in a garage?

That depends entirely on what a 'garge' is.

If you are asking whether or not they can sleep in a garage then that would depend on the environment within. Guinea pigs should be kept in a relatively warm (no colder than 15 degrees Celsius) environment . The temperature should not get too hot, though― too much more than 25°C and your poor little cavy runs the risk of heat exhaustion. He's also going to need some soft bedding (think wood chips).

If the garage is climate-controlled, this should not be a problem, but most unfinished garages are too hot or too cold for a guinea pig to live comfortably in.

If you wouldn't be comfortable sleeping in the garage, your guinea pig won't be, either. Better to keep him in a proper pen inside the house.