I've had enough time (and some really hot weather) to do some comparison test between my Leaf and a relative (driving a 2015, Gen 1). This is the conclusion I've come up with, totally opinion of course with the limited measuring abilities I have to work with.
I'll start with the 2015 Leaf. For anyone new to the generation names we use here. Usually Generation 1 Leaf are those that have either the 24 kWh or 30 kWh battery packs. This usually covers anything 2017 and older models. I am aware that the replacement batteries for the 30 kWh packs are actually 40 kWh packs *now*, but basically the classic Leaf body design is what I am referring to.
The Gen 1 Leaf is basically limited to 80 kW of power maximum for driving. The gear box is just a single speed reduction gear set that is filled with 1.5 Quarts of Nissan Matic S gear oil from the factory. The Gen 1 Leaf I was experimenting on has a temperature point in the electronics and motor that will switch on the both radiator fans and put the coolant pump into high mode. This temperature appears to be around 140F - 142F (60C - 61C). When the motor+electronics get this hot, an additional power draw of between 600 to 700 watts is used to cool it down. In the grand scheme of things, this is a small percentage of the power used to drive the vehicle.
Knowing that information, how would changing the gear oil have any effect on this? It's actually an interesting case of keeping the motor just cool enough *NOT* to trigger this extra cooling mode. That is where the gear oil change comes in. When the Leaf comes new from the factory, the gears have some "break in" time where they polish each other and in the process, chip off little bits of metal that float around in the gear oil. The Leaf isn't driving an automatic transmission, so those little bits usually get caught by the two magnets inside the gear housing and don't cause any problems. The even tinier bits though that are not magnetic or others that are rust, they diffuse into the gear oil and turn it black basically. The gear oil still functions like it should, but the viscosity is raised a little as more time passes.
Eventually, this increase in viscosity will make the thicker gear oil produce more friction when driving and thus produces more heat. Remember, in the grand scheme of things, this isn't a terrible energy loss. You still drive the Leaf and everything is OK mechanically. This is only 1.5 Quarts of liquid I am referring to. Now, picture this. The Leaf Motor, Electronics, Gears, etc. are all connected to a single "box" of metal. This means that heat can transfer through out the box, which is also fine. That is how the coolant lines keep everything cool. Remember that the gear oil is a source of heat being generated by the friction of gears while driving. That heat will be carried away by the cooling system eventually.
From what I have been measuring, the temperature of the gear oil will eventually surprise the temperature of the motor. When it does, the cooling system must spend power to keep the temperature down. What if you could keep the temperature of the gear oil down low enough as to not trigger the "high" cooling mode of the Leaf and the power penalty that goes with it? That is benefit that I have been recording and watching when doing a gear oil change to ULV (Ultra Low Viscosity) gear oil. This benefit is not limited to just ULV though as I've seen a marginal increase by just changing out the gear oil with the same stuff that Nissan uses to begin with.
The key to it all is the temperature. If you have the ability to use LeafSpy with your Leaf, then on the next drive, watch your motor temperature and see if it frequently goes above 140F. Once it goes above 140F, you'll know that those radiator fans are probably running and your coolant pump is probably running in high mode. If you can keep the temperature down below this, this is where you will see an increase in your range, long term.
My conclusion, changing the gear oil, even with the default Nissan Matic S has a benefit if the existing gear oil was very dirty for Gen1 or Gen2 Leaf. I was going to make a large chart of numbers, but they all depend on your driving style and thus don't really fit all conditions or ways you may drive. Maybe the gear oil has never been changed in the life of vehicle? You will see a benefit, especially if it has over 10k miles. Want to experiment and go for a bigger benefit? You can try other gear oils that have a lower viscosity or better chemistry than the default Nissan uses, but just keep in mind that there is some inherit risk doing this. I would suggest reading over this whole topic and not just the end here to learn about the risk or using gear oil that is *too* thin.
If your driving style is constantly keeping the motor above 140F (60C), then a gear oil change might benefit you. If your driving style is always keeping the motor at 190F (87C), then you probably won't notice much of a benefit to the gear oil change. The variables are too complex to say that a gear oil change equals instant range increase, but it won't hurt to try.