Many people who support the Trump Wall do not support gun control. The main argument against gun control is that bad guys don't follow the law so they'll find another means in obtaining a firearm. By that same logic, building a wall will not stop by bad people from drug trafficking, sex trafficking, or human trafficking. It might stop some but you are naive if you think this will inhibit the cartels from doing their business.
The differences between the two is that gun control is usually brought up after some crazy shoots up a public space and usually these act on their own accord. While drug, sex, and human trafficking is a multi billion dollar industry that involves many, many more than just 1 person. Its much easier to place limitations that are directed at inhibiting a single person than it is to stop a much larger (and financially backed) group of people.
So I find it ridiculous that people think gun control wont slow down mass shootings but building a wall is going to slow down a billion dollar industry.