3f339d4d_Gerbil_Care__It_might_be___
[My Gerbil...] has lost part of his tail! There's blood and bone showing! [It might be...] 
your gerbil has degloved his tail. Try to judge if your gerbil is in pain. If she is squeaking, hiding, nips you, or has her ears plastered back, she probably is in pain, and you should see the vet for an amputation. It can be expensive, but it will solve the problem right away, and your vet will give her antibiotics. If your gerbil seems completely normal, the wound will eventually heal. The exposed bone will fall off. However, you must keep the wound clean. Wash it and apply antibiotic ointment twice a day. Find a bedding that won't stick to the wound but will be soft. Torn up tissue paper might be best, or shredded brown paper Eco-Bedding. Or you can shred paper yourself with a home shredder. Change it frequently to minimize infection, and put your gerbil on a course of emergency ornacycline treatment.
Gerbil Care
It might be...