I've killed a quite a few gnomes, and there are corpses all over the place. I can't pick them up without being encumbered. Is there anything useful I can do with them?
There are a couple of options that spring immediately to mind.
You could eat them. This risks sickness. Feeding them to your pet is also an option.
Or you could sacrifice (offer) them to your God, if there happens to a be an altar nearby.
There are likely other options - but those are the two that spring immediately to mind.
Different corpses give different benefits/hinderances. Looking at a NetHack wiki shows some can be hallucinegens, some poisons, etc.
Polypiling them is an option.
Since they are considered food, they may change in to other food, or disappear.
If you have enough of them in one pile, they may become a flesh golem, which may provide a 12% resistance to cold, poison, shock, or fire.