Assuming a minimal gravity to retain a life-sustaining atmosphere does not make things easier!
Retaining an atmosphere requires a certain minimum escape velocity, and that's exactly what you want to minimize. Life, fortunately, does not require an atmosphere. Life requires liquid, but deep ocean life would get along just fine if there were no atmosphere above it, and plenty of organisms (famously including tardigrades) can survive in stasis in vacuum. Humans can even operate for up to 90 seconds (in the ideal case) in vacuum without permanent injury, so it's not that hard to imagine an organism that has developed in an ocean (or at least, say, a sub-surface lake) and then evolved full resistance to vacuum, maintaining its own internal liquid environment. Really, it's not that much different from Earthling creatures moving from the ocean onto land, and developing impermeable skin to take our watery environment with us.
Liquid environments can exist under icy or rocky shells, with no need for an atmosphere, on bodies with much lower escape velocities. The ideal environment for evolving an organism that naturally propels into space is, therefore, probably something like a small ice moon like Enceladus--which does, in fact, have geysers that expel material above escape velocity--or a large cometary / Kuiper-belt object like Pluto or Arrokoth--large enough to have significant internal heat, and retain liquid underneath an ice shell, but small enough to easily escape from.
Robert Forward describes this kind of organism in Camelot 30K. (Spoilers for a 29-year-old SF novel incoming.) In that novel, intelligent life is discovered on a Kuiper-belt object at 30K. The animal-analogs are endotherms, which actively maintain their own pressurized internal liquid environment against the low-temperature, hard vacuum environment of the world's surface. The ecosystem survives on the energy of reactive chemical radicals left in the ice by the passage of cosmic rays, and the heat released by bioaccumulating radionuclides.
The power input from cosmic rays is not huge, and radionuclides are rare, so they are using up the energetic resources of their tiny world far more quickly than they will be replenished--which means that they cannot survive indefinitely on just one world. They must travel between low-gravity icy bodies. And in fact, it turns out that every organism in the alien "ecosystem" is actually genetically identical--the plants, animals, people, everything, are actually just clones of a single organism, exhibiting extreme phenotypic plasticity (like different casts of ants) in order to instinctively build a structure that will launch their colony into space--which kills most of the individuals, but allows fertile spores to drift to other bodies where they can germinate and start the cycle over again.