Is there a trick to filling a gap between flooring and a door jamb? Previous floor was thicker. Wood filler and paint ?

As I see it, you have a few choices. Replace the jamb with a new one that fits or fill the gap at the bottom. If it is very small, paintable caulking will work. For larger gaps, either cut and glue a piece of wood there or use a quality filler like Bondo that will get hard and avoid shrinking and falling out. Then paint the area. That's how I see it. Either way, kind of a hassle, to be sure.