Non-smokers are rallying in support of the new no-smoking law.
Literal
Non-smokers [topic-は] new no-smoking law [object-を] support [then-して] rallying.
非喫煙者 uses the prefix 非 ('non-') + 喫煙者 ('smoker'). 禁煙法 is 'no-smoking law.' 結集する means 'to rally, band together.' ~して links the purpose (supporting) to the action (rallying).