Healing professions are doing all they can to curb gun violence. Unfortunately, they only have a small amount of power and influence in the grand scheme of things. They cannot monitor everyone and can't do much if some random person wakes up one morning and decides he is going to shoot someone or something.
It is not the responsibility of healing professionals to stop gun violence. Frankly, there isn't much they could do to stop a lunatic from going over the edge and acting out. Psychology is not a science and all one can do is to treat using the best instincts, and the best available known methods.
Healing professionals are not in the position to control things that rightfully should be controlled at another level. A doctor or psychiatrist's job is to treat the patient and they are further bound by confidentiality laws. It often comes as a surprise to everyone when somebody becomes violent. I don't know what the health profession could do more, and this question just tries to put the blame on gun violence one step removed from their violent patient. We need to stop looking for people to blame and start blaming the access to guns.
There are a lot of things that need to be addressed to get this under control, but it starts with mental health. Mental illness is demonized which makes many carrying some form of it hesitant to get treatment, and even if they do, it isn't funded heavily enough to ensure their needs will be met. They want to do enough, they are not given the means to do so.