Sure, you can do it right now actually. But only at the lowest end on the latest games that are difficult to render, not moderate. But should be moderate on average to older games. And a sprinkle of high settings on some games.
If you look at historical trend, with respect to gaming PC's, the power consumption is going up (more so at the top end) with discrete GPU's involved. There was a spike as an outlier when SLI was a "thing" for a moment (with two GPUs drawing lots of current), but it's long dead and replaced with single monstrous GPUs now that have massive current draw. There have been good lower power GPU's that didn't even require a PCIe power rail to them (like the 750Ti, etc). But even those cards draw up to 75 watts from the board itself which puts it into a higher power draw category overall after you factor in the board itself, components and CPU at load, it will still easily be a 150~200 watt draw of current. That's not 100 watts or less, so that puts it over your defined limit. The moment you use a GPU with a PCIe rail, it's because it's going to draw more than 75 watts, basically, as the board itself will not supply more than that typically. So the mid-tier GPU's seem to draw anywhere from 125~300watts of current at load. And it goes up from there. It's also going up on average because components for a moderate to high end gaming PC are more and more affordable than they were many years ago. CPU current draw and overall motherboard draw is not to terribly different than it ever was and in some cases are less, thus higher efficiency these days. But the GPU trend is power consumption is very high still and I don't see that going down any time soon.
So to make a low power gaming PC, you basically will not be able to use a high current draw GPU. That puts a lot of limits and compromises on the build for gaming because the GPU is the heart and soul of gaming.
That said, the APU comes into play. Today's APU makes it possible to build a low end gaming machine that will play most games at 1080p with medium to low settings on new stuff, and high settings on older stuff. So the Ryzen 3400G for example is a 4c/8t CPU with Vega 11 GPU in there, together, at 65 watts at full load. There's not another CPU + GPU combo that will beat 65 watts total power at load with something discrete because even a low 35 watt CPU plus a 50~60 watt low end discreet GPU (which will not even compete with Vega) is still higher than 65 watts. So I'm not sure if there's anything that will beat 65 watts for a CPU + GPU combo at load.
So if the Ryzen 3400G at full load for the CPU + GPU hits 65 watts max (and often isn't at that exactly, it's lower too, that's just peak max), you still have to factor in the motherboard and it's components (RAM, etc) which will all draw some power too. Then there's the efficiency factor for converting actual wall draw and the consumed current and the rest is lost as heat. So ideally you would want the most efficient PSU you can get, to not draw more power to make up for the thermal loss of lower efficiency. You can still expect total system draw at the wall to be closer to 120~130 watts total for the PC itself. That does not include your monitor and speakers, etc. This is likely as close as you'll get to a 100 watt moderate gaming PC currently.
For reference, the latest 4k consoles draw close to this and a bit less in some cases. It's not significant enough to justify the compromise though since the PC will be more flexible for everything. But if your goal is pure low power, a console currently will likely hold the title for lowest power consuming moderate gaming, as long as gaming is the only thing you care about and don't mind it being uniquely console games.
Very best,