Not really. That would be like claiming it’s “naturalism of the gaps” if scientists “assume” there’s a natural explanation for how some pollutant causes cancer.
It’s more a case of the history of science having already been so overwhelmingly successful at finding natural, physical explanations for so many other aspects of the natural world (not just life), such as evolution, that it would be perverse to deny this history.
Including but not limited to the material bases of all other aspects of life, such as the need to ingest physical food and chemically turn it into the constituents of the physical body, and to obtain energy to drive the physical processes of life such as growth and reproduction, the physical basis of all the senses, etc. etc.
This historical evidence from the chemical and physical sciences all strongly point in the same direction. Life’s ultimate origins is most likely to be another natural phenomenon.
And then there’s the fact that origin of life research has given a few interesting clues pointing to life’s origins having a physical and chemical basis.
Nothing about this is gap-reasoning. It’s entirely reasonable abductive and inductive reasoning.