OP, can you clarify whether you mean kennel (as in a large enclosed space, such as a small dog pen) or crate (big enough for pup to lay down in, but not move around much otherwise)?
If it's an actual kennel (a pen) that you have in the garage, I can understand why you would be keeping pup in the kennel at night vs loose in the house. However, you could simply get a crate for inside the house if this is the only issue.
You won't find many people here who will approve of keeping a dog in the garage at night, or outside. Most of the members here are housepet kind of dog owners. That said, german shepherds certainly will be fine in those temps with some warm bedding as another poster suggested. Unless you plan to put the puppy beside your bed in a crate, there is no difference IMHO whether the pup sleeps in a crate in the house or sleeps in a crate/kennel in the garage. Your garage, if attached to the house, probably stays much warmer than 20 degrees (ours never goes below about 45 degrees at the coldest even in the dead of winter, and I'm in central Indiana). Also, the garage is out of the wind and other elements, so even if it truely is 20, it's not as cold of a 20 as 20 degrees outside will feel. In order to stay outside in those temps, the pup would have to be acclimated to outdoor temps already by then which would mean living outside 24/7.
In short, yes, sleeping in the garage is fine. It will not hurt the puppy any, will not hurt bonding, or anything else. Plently of people are bonded to their dogs. Most working dogs (I mean actual working dogs, not sport dogs) live outside and are VERY bonded to their handlers. Where the dog sleeps has nothing to do with bonding. What you do during the awake hours is what promotes bonding.