Zwift has a “trainer difficulty” setting (other apps often do too) that effectively changes your gearing for the inclines. The max gradient I’m aware of, albeit very brief, is 21% on the Yorkshire reverse circuit. A few routes top out at ~17%, but even “flat” routes often throw in short inclines that might hit ~8%, such as Innsbrookring.
Your speed is determined by power and weight (plus drafting if not on TT bike) in Zwift, so a rider putting out 4W/Kg at 100% trainer difficulty on a TT bike would get up a climb faster than another rider putting out say 3.5W/Kg at 75% trainer difficulty.
My rear derailleur cable recently snapped, so with the rear derailleur “high” screw, I’m stuck in the 16T sprocket of my 11-32 cassette to use with my 34/50 rings. I was happy to use 100% “trainer difficulty” on Volcano CCW yesterday, which pushed me close to my limits at ~500W up the incline towards the end of the lap (~5%), but I turned it down to ~40% for Innsbrookring for the race I joined ~20mins later and doing ~300W up that incline killed my legs off.
Each turbo model has a unique power range for each gear combo, depending upon cadence. For example, 34/18 at ~75rpm has a range of 84-484W IIRC.