They could only really enforce this for registered guns, otherwise how will they know who does or doesn't have guns, and how many?
So, the people with registered guns, a group that I imagine is largely law abiding (in this regard) would have to turn in their guns.
All the gangbangers, criminals etc. who get their guns via the black market, would be able to keep their guns. They are already criminal, so why would they voluntarily hand in guns that the government wouldn't otherwise know about?
I don't think a policy to take away newly outlawed guns would be a very good idea, and I imagine it would make society much more dangerous, even if everyone were to voluntarily comply, which is the best case scenario.
Realistically, the only way to outlaw guns would be to prohibit, sales, transfers etc. of firearms, not just try to "disarm" the public. Getting a gun free society like this would take many generations of course, and could never be fully done.
It's best just to leave the gun laws the way they are (for the most part). Try to keep track of the guns but don't outlaw them.