There are venting requirements for every fuel burning piece of equipment. Most every local in the country is under some form of code and most local codes refer to venting codes in one form or another. Sometimes they just do a blanket statement of "installation must conform to manufacturers recommendations".
So...... short of saying yes you need a liner...... I'd be willing to bet you will.
GAMA is national and local codes must exceed GAMA to be effective. This poster is in New Jersey so most likely the GAMA venting charts would require the smaller venting.