I agree, the vast majority of gun violence (as if guns are violent all by themselves) are not the result of mental illness, but the vast majority of mass shootings are done by people who have been heavily medicated for years. The over prescribing of meds for just about everything is evident all over. I know people in my own family who are on a wide range of meds from high blood pressure to anti-depressants, they’re like a walking pharmacy with their ever present bag of meds.
The buzzword "gun violence" was created by politicians and the media to mask the real roots of the problem. After all, nobody refers to stabbings as "knife violence" or vehicular homicide as "automobile violence" or maintains the pretense that the instruments used are to blame.
What politicians and media call "gun violence" actually falls into two categories. Mass shooters are almost invariably mentally ill, but they account for a small fraction of shootings. The overwhelming majority of the remaining shootings are gang-related - Crips vs. Bloods, Zetas vs. Aztecas, etc. It's much more politically correct to refer to the latter as "gun violence" by blaming the tools than going after the culture and mindset that actually creates the problem.