In terms of gross morphology? No idea. How thick is the atmosphere? How far is it from its sun? If we knew how dense the atmosphere was, we might be able to says something about valid options for flying and other forms of locomotion. Given the easy availability of hydrogen in a nitrogen-dominated atmosphere, if the air is thick enough, it might make aerostatic flight (balloon creatures) an easy and obvious evolutionary possibility. Also, hydrogen is a pretty good greenhouse gas at high pressures, and the farther it is from the sun, the less significant the temperature swings from orbiting the gas giant at such a huge distance would be. Still, unless you simply made a typo in how many zeroes there are in distance from the gas giant, it seems likely you'll have a Cycle of Fire kind of scenario, with the world alternately boiling and freezing, normal life proceeding in the temperate periods in between and needing to survive as spores in between.
But otherwise (assuming that macroscopic life is possible with that atmosphere), macroscopic life would be not be obviously more or less constrained than it is on Earth. And Earth has produced a lot of really weird stuff. Lower gravity means animals and plants could be larger and/or more gracile, all other things being equal, than they are here on Earth, but unless you're in this world's age of gigantic dinosaurs, that won't make that much difference. There's a slightly higher chance that land creatures on this world would have more legs, in order to get more traction in lower gravity, but that's not a guarantee, either.
The interesting stuff is in biochemistry, and how that influences ecology- the "how they interact" stuff.
If we assume they breathe hydrogen, they probably wouldn't need as efficient gas-exchange structures (gills or complex lungs) or gas-transport structures (haemoglobin, red blood cells) as oxygen-breathers, since hydrogen is much smaller and diffuses easier than oxygen. Hydrogen-based respiration is only about 4 or 5 times less efficient than oxygen-based respiration, rather than the factor of 30 that purely anaerobic respiration lags, so that doesn't seem like quite as big a deal- and remember that that's for food molecules that evolved in our environment. Organisms on this world would likely evolve energy storage mechanisms optimized for higher chemical energy densities in their environment. On the other side of the energy equation Photosynthesis in Hydrogen Dominated Atmospheres actually requires less energy than oxygen photosynthesis (balancing the lower amount of energy that heterotrophs get by reversing it), and is likely easier to evolve; thus, it may actually be easier to get large, complex autotrophs (i.e., plants) on this planet than it was on ours.
If, however, the heterotrophs aren't hydrogen breathers, then the ecology probably works something like it does in The Nitrogen Fix, with heterotrophs not needing to breathe at all, and either consuming both oxidizing and reducing food molecules in solid or liquid form, or else running purely on decomposition reactions of high-energy molecules. That doesn't work too great for Earthlings, but remember that we're optimized for a relatively higher temperature, oxidizing environment. Especially given the ready accessibility of bio-available nitrogen in the form of ammonia on this world, they may use much higher energy density complex nitrogen molecules, such as would dangerous explosives in our environment, as energy storage rather than, or in addition to, hydrocarbons and carbohydrates.
Now, if this life is carbon based, it'll need a carbon source. That's not a huge deal- we can assume there's a trace amount of methane in the atmosphere, part of that 0.5% other substances, analogous to our tiny trace amount of carbon dioxide, which serves as the primary carbon source for Earthling plants. If that's not present, however, the ecosystem would need to extract carbon from crustal rocks, which would mean that plants don't breathe, you probably have a much more important and extensive network of fungus-analog symbiotes breaking down rock to extract carbon, and the landscape would be broken down into soil at a much faster rate relative to metabolism than it is on Earth. On the other hand, though, once life gets ahold of it, it seems very likely that the ecology would end up releasing methane into the atmosphere, and then this isn't such a big deal anymore.
If life on this world isn't primarily carbon-based, then that's not really relevant anyway, but could that actually happen? I don't know. I've seen serious proposals for life based on complex nitrogen-phosphorus chemistry, with N-P pairs replacing single carbon atoms, so maybe you could do the same with nitrogen-boron chemistry? I'd buy it in a sci-fi novel, anyway. But sticking with carbon-based life using trace methane as a carbon source seems like a much safer bet.
Even if this world's life doesn't rely on carbohydrates for structure or energy storage, they will still probably need an oxygen source as well, just because oxygen is such a useful element in all sorts of complex compounds. Perhaps, at cryogenic temperatures, they replace disulfide bridges with dioxygen bridges. Like the carbon source, this is fairly easy to arrange- there's probably a trace amount of water (or "oxic acid", as the natives would say) dissolved in the ammonia oceans, and weatherable out of rock. Given the difficulty of splitting water (which is part of what makes our own oxygenic photosynthesis so energy intensive), it seems likely that there would be specialized, symbiotic oxygen-fixing microbes, analogous to our own nitrogen-fixing microbes, which serve to split water and produce molecules providing oxygen in a more bio-available form to other organisms.
And, of course, you could hardly pass up the opportunity for creatures with shells or bones made of nitrogen boride. :)