1. mountain bike trails: mix of relatively flat, hardpack trails but lots of short, punchy climbs, nasty exposed limestone, sometimes ledgy with scree-strewn slopes. I can ride it all on the same bike.
"gravel" terrain: above mentioned hardpack trails, suburban roads, crushed granite trails, rural dirt roads when I get out of town.
2. mountain bike: steel hardtail with 29" tires and a 120mm suspension fork. fairly heavy but durable.
gravel bike: steel frame and carbon fork, drop bar, disc brakes, 650b 47mm tubeless tires
3. rolling hills on the road, punchy climbs on the trails
4. mtb: 30/20 single speed. this is just a bit low, but with the track ends, this allows me to push my axle almost all the way forward because I prefer a short rear-center. ideal gear would be 33/21 but odd-tooth count sized rings are not common for my crankset. I can make it work but I've not bothered with it. the low gear is a bit spinny on the flats but my goal is to make the punchy, technical climbs with only one gear choice.
gravel: 42/18 singlespeed with a eccentric bottom bracket. I had it at 42/17 and that was starting to get a bit rough on the knees. I am not crit racing on this bike, just exploring and this gear is just right for pushing a nice pace on flat roads but does not stop me from climbing long, steep roads. if you look up "Tour Das Hugel" you can see the range of road climbs where I life.
5. I've got these where I want them for now. having a few cogs in my parts bin allows me to change them as needed.