Gearing in ERG Mode
Technically, if you are doing target power training, gear selection does not affect your training. 260w is 260w in any gear. But an easier gear combo has slightly different leverage action from harder gears, and when you ride in a harder gear your flywheel has greater momentum which also factors in.
There is some validity to using appropriate gearing on the Erg trainer to better target different muscles that are triggered by different gearing. The short answer is “Ride in gears that simulate what you’re riding is like outside.”
Although I mostly use Trainer Road and do structured training, I also use VirtuGo (like Zwift) and Zwift in freeride mode so I can ride around and practice changing gears specifically for climbing. It’s an excellent way to understand your gearing combos and your wattage output so when you get outside you can ‘replay’ what is working from your training. (I don’t have power meters on my mtb bikes). I have also started shifting to harder gears when doing low cadence high power intervals so I don’t have to slow my cadence early in erg mode. It feels right to me and my Kickr reacts very quickly so it seems to be good. I keep power smoothing on most of the time so I don’t notice the power jumps unless I dig into the workout data.
Hope this helps.