Get that Gramma out of your tank!
They're peaceful if no one comes near his spot, he's VERY territorial. If your tank isnt big enough for other fish to swim WAY past his spot he's going to chase/attack/bully/hassle (pick yours) them.
I've had them in a 13.5G nano and the only fish that didn't get hassled were the clowns, because they'll kick his butt if he does. I had to get rid of him since if he didnt kill a fish directly, his behavior would cause other fish to be too stressed and hide, then they wont eat and then they get "sick" then they die.
Then I tried him on my 90G and still, he would just claim to much space as his and therefore disrupt the entire swimming pattern of the other fish, not to mention the tension "in the water", you can feel it. Eventually I donated him quickly.
I also fell for the "peaceful" category in Liveaquaria and they're correct in a sense that they could not classify the gramma as semi-agressive, because territorial isnt exactly the same as semi-agressive. Semi-agressive fish can be really agressive, beyond territorial.
I learned that there are some nuances in the subjective meaning of peaceful, there could be specific circumstances that will influence their behavior for good or worse.
There are other fish that are also classified as peaceful but DEPENDING on your setup they might not be suited. I've had midas blenny kill a couple of fish indirectly, and I tried quite a few due to personality traits that also might influence behavior. They're also classified as peaceful.
So, although the Gramma is classified as peaceful and SEEMS TO BE one of the most beautiful fish suitable for a small peaceful tank, he's not worth it, unfortunately. Accept it, choose something else, keep your tank peaceful, your fish healthy and happy.
I would recommend him for a semi-agressive tank full of damsels and dottybacks that behave more like him.
I was never able to recover a wounded fish, very hard, requires a lot of knowledge and experience, so don't feel bad. Just learn the lesson and move on.
Cheer up!