I just replaced the front bumper rub strip with a "Brand new one" which I purchased from my Buick dealer. It is unpainted black (Kind of dull black color but dosn't look bad). The back one is a gloss black, looks as though it has been painted. Question! What was the orginal factory finish on the black body rub strips, Gloss or the unpainted natural black?

They are painted gloss black, unless they are painted body color. The finish should match the body side moldings. I went through the same thing about a year ago. I had the bodyshop paint the front molding before I installed it. You probably can paint it yourself but you must use the right type of paint.
Mark

SEM makes gloss (and satin for that matter...mirrors and cowl cover) black trim paint in a spray can that was formulated for this type of application. May be obtained from most automotive paint jobbers or google Vinyl Pro of Western PA.