I'm going to be brutally honest and blunt, and the answer may not be very widely accepted... However, any time I encounter an injured snake in such condition, it's in a lot of pain most likely, in my opinion, the best thing you can do for the animal, is put it down to stop said pain.

If a garter is that damaged, odds of it surviving and doing well without extensive vet work, lots of money, time, and pain involved on the animals end, is simply not worth it in my eyes.

-Mike