I've been looking at this issue with regard to Codes of Conduct. These come in many flavors: for companies, for conferences, for students and universities.
Many corporations explicitly forbid carrying a firearm onto the premises. Sometimes there's contention w/r to the parking lot and whether a personal vehicle should be considered a permitted place to store one's gun while working. Some companies ban this practice saying the parking lot is theirs and their policy extends to all of their properties. Exceptions are made only for security guards, police etc.
Clearly a conference hosted by a university or company would not be allowed to circumvent the code of that institution, or would it? If an outside group rents space at a college or university during the summer, does the student code of conduct apply to them? If a company donates space to a non-profit for a meeting, can it enforce a no-carry rule even in a state that permits carrying?
Restaurants reserve the right to refuse service to anyone (so says the sign).
"No shirt, no shoes, no service" is common.
So what about "Got a gun? Keep out. Private Property."
Can you post that on a hotel or restaurant door? There seems to be some consensus, among USA citizens at least, that such signs may be safely (legally) ignored, at least in states that permit open or concealed carrying.
The issue is gripping many in the USA not just because of school shootings (which have been many) but because of the new generation of "stand your ground" laws, which appear to make it easier to claim self defense even when it turns out the only person with a gun at the scene considered themselves a potential victim, while the person ending up dead was not carrying a weapon ("how was I supposed to know?").
These issues are germane in that the Python Software Foundation has been developing its code of conduct for its international conferences. Obviously "one size fits all" need not be a goal.
We have gotten as far as stipulating that for PSF to be involved in a supportive role, *some* code of conduct needs to be on the books, but the details may be left to the locale (and need not be in the English language, though translations would make sense if the conference is truly international).
Here's the US Pycon code of conduct (current version). Nothing about guns though. More like the Atheist thing.