'banning' in your sentence is a 'present participle'. (And I agree with you that 'smoking' is a noun (gerund).)
"Laws (banning smoking in public places) are about to be passed."
Those words in parentheses (which I think are called a 'participle clause') above postmodify the noun 'Laws'. They describe what kind of 'laws' they are. Therefore they function adjectivally. It is very much like a relative clause:
"Laws (which ban smoking in public places) are about to be passed."
I hope you'll understand what I mean.
(I'm sorry that I cannot explain well enough how to distinguish the difference between 'participles' and 'gerunds'.)