Geckos can sustain injuries from various causes. Possible causes: they are self inflicted, sometimes a cage mate gets too aggressive, sharp or abrasive cage furnishings, crickets left in the cage deciding to chew on the gecko, and bad sheds or improper humidity causing bacterial or fungal infections.
Nemo's Story: The injury you see in the picture above is pretty severe, it is also part way healed and at the same time at a critical point in healing-because it has become infected. The yellow/brown is not a healthy color for a reptiles injury. I purchased the gecko already injured so I can only guess at the cause. He was being 'gecko sat' by a reptile keeper other than the owner. He was missing the toes on his left foot and the skin on his foot and hemipene/tail area was missing as well. This gecko had been taken to the vet when the injury was first discovered by the gecko sitter. He had been placed on oral antibiotics and the recommendation was to wash the area with a disinfectant such as chlorhexidine. At the time the picture was taken he had finished the oral anti-biotics and infection had started in despite disinfectant and a sterile environment. I started using a topical triple antibiotic in conjunction with the disinfectant.
I cleaned the injury twice a day and applied the antibiotic. Within a week the injury had turned color as the infection went away and the healthy new grey skin started covering the area. This would not be a healthy color for a human, but it looks great on a reptile. The scales haven't started growing back on the area yet so it doesn't show color.
The scales grew back over the new skin within the month. Nemo was eating like a pig at this point and gained 20 grams. He went from 35 grams when he arrived to 55 grams. Today he hovers around 70.