The declaration of emergency lets trump use funds the mil has designated for construction to be used for constructing a different structures (like the wall, supposedly).
I'm not sure how this could be used for gun bans, since it is just shuffling construction funds around.
If I am wrong, I would sure like to know.
One thing I do know is the Republican party's refusal to fund the wall when they had power was absolutely disgraceful.
Pelosi and "We'll do it for guns" is, IMO, more of a scare tactic. Guns would be much harder to do because they're mentioned as the second item in that whatchyamacallit document.
They could much more easily do "climate change" though, using the identical arguments Trump is using, and the same methods to pay for it. And unlike a wall at the border, that "emergency" would negatively affect every single person in the United States. Well, except for alternative power companies, enviro NGOs, and politicians with ties to them.