This is kind of an interesting thought experiment around laws in general. If you made guns illegal tomorrow you would certainly increase guns crimes because anyone who doesn't give up their guns is now committing a crime. Similarly all sales of guns would now be illegal and the demand for illegal guns would go way up resulting in more black market trade.

But the framing of the question leads to that conclusion. If what you are actually after is something more akin to "if legalizing drugs decreases violent crime then shouldn't legalizing guns also decrease violent crime?" This is more interesting in my opinion.

Now let's assume for a second that legalizing drugs does decrease violent crime (I do believe there I'd research that supports this). What crime is reduced? It is likely that crime around the drug trade itself is what is mostly reduced: you do not have as much cartel, gang, mob, and other organized crime entities around who are committing much of the violent crime. Drug users themselves are probably committing less of the crime than the actual distribution as using drugs is often times not an inherently violent act.

For guns the gun itself is a tool of violence. You cannot use a gun without committing a violent act. Now not all violent acts are violent crime, as target practice is not exactly illegal, but it is still violent. If you make manufacturing of guns illegal less will be manufactured which means less will be in existence therefore there is inherently less access to tools of violence. I'm sure the drug smuggling trade would introduce more violence as it would increase drastically. However the increase there may be smaller than the decrease of just having less guns in the world.

I guess the key difference is that guns are a tool of violence much more frequently than the use of an illicit substance is most likely.