Most living things produce carbon dioxide as a byproduct of breaking down molecules like sugar for energy. Things that do photosynthesis produce oxygen as a byproduct when light is available. When it's not, they produce carbon dioxide. Neither carbon dioxide nor oxygen have a smell that humans detect.

Animals eat and digest food. A lot of the digestion is actually done by bacteria that live in the guts of animals. These bacteria produce gases such as methane, which is smelly.

Bacteria produce a lot of the gases that we think of as smelly. For example, when food spoils or bodies decompose, it's the action of bacteria that make the smelly gases.

Whether we actually smell something mostly depends on whether those chemicals fit into receptors in our nose. These receptors send messages to the brain where we not only notice the smell, but may have a strong emotional reaction to it.

Do you think our responses to odors--whether we find them pleasant or unpleasant--is instinct, learned, or both?

Gases have odors if they interact with structures in our nose that are involved with smell. Once I worked for a professor who used a special molecule that smelled like popcorn. The bottle said the molecule had 'no physiological effect' but the professor said that was wrong, because being able to smell the chemical was a physiological effect.

This just has the 2 obvious gases:
gases in water .

The two essential gases for living organisms are oxygen and carbon dioxide (CO₂). These two gases are found in the air as well as in the water. Land-living organisms, as humans, breathe oxygen from the air; fish breathe oxygen from the water. Carbon dioxide is produced by animals and humans.

Farts are primarily made of odorless vapors like carbon dioxide, oxygen, nitrogen, hydrogen, and sometimes methane. Although gas is a normal part of life, it can be inconvenient. You can't stop farting completely, but there are ways to reduce the amount of gas in your system.

Gas and smell.

Those are all disappointing answers. If you want to know a LOT, read 'Nose Dive. A Field Guide to the World's Smells' by Harold McGee. Part 2 is about animal smells. Part 3 is about smells of land plants.

Here's a 5-min audio and transcript of an interview with Harold McGee: nose dive.

The ultimate guide to the smells of the universe – the ambrosial to the malodorous, and everything in between – from the author of the acclaimed culinary guides On Food and Cooking and Keys to Good Cooking.

"From Harold McGee, James Beard Award-winning author and leading expert on the science of food and cooking, comes an extensive exploration of the long-overlooked world of smell. In Nose Dive, McGee takes us on a sensory adventure, from the sulfurous nascent earth more than four billion years ago, to the fruit-filled Tian Shan mountain range north of the Himalayas, to the keyboard of your laptop, where trace notes of phenol and formaldehyde escape between the keys. We'll sniff the ordinary (wet pavement and cut grass) and the extraordinary (ambergris and truffles), the delightful (roses and vanilla) and the challenging (swamplands and durians). We'll smell one another. We'll smell ourselves.

Through it all, McGee familiarizes us with the actual bits of matter that we breathe in—the molecules that trigger our perceptions, that prompt the citrusy smells of coriander and beer and the medicinal smells of daffodils and sea urchins. And like everything in the physical world, molecules have histories. Many of the molecules that we smell every day existed long before any creature was around to smell them—before there was even a planet for those creatures to live on. Beginning with the origins of those molecules in interstellar space, McGee moves onward through the smells of our planet, the air and the oceans, the forest and the meadows and the city, all the way to the smells of incense, perfume, wine, and food.

Here is a story of the world, of every smell under our collective nose. A work of astounding scholarship and originality, Nose Dive distills the science behind the smells and translates it, as only McGee can, into an accessible and entertaining guide. Incorporating the latest insights of biology and chemistry, and interweaving them with personal observations, he reveals how our sense of smell has the power to expose invisible, intangible details of our material world and trigger in us feelings that are the very essence of being alive."

There are probably too many to list. Prominent examples of gasses produced by living things include oxygen, carbon dioxide, water vapor, and methane.

Gasses have odors if they trigger chemical sensors in your nose. Water vapor is the only one of these gasses that I know to have an odor, which you can notice when it has recently been raining.