Why is a gastropod's shell not consid-
ered an exoskeleton?

Gastropod's shell not considered an exoskeleton because they do not shed their exoskeleton.

Explanation:

Gastropods like snails have unique shell structure which protects them with the hard covering and by providing a camouflage appearance.

The shells of the gastropods contain protein and calcium which facilitates growth of the shells. The gastropod shells often grow from bottom to top layering into three layers – the inner nacre, prism, and the periosteal layer.

The exoskeleton of animals like the arthropods exclusively contains the polysaccharide chitin. The exoskeleton will periodically shed because it is acellular and does not grow in line with the growth of the organism.  

it's anything that aren't producers and consumers.

explanation:

you've got to be clear on what "this" is