The built in stabilisation is surprisingly good!
Most of my town is built on what was a huge private estate, and one of the estate roads was an ornamental drive. Some of this drive is heathland paths, and a portion is built on. I'm making a film tracing the road as it is today with a commentary about the history. Mostly osmo footage, with other stuff for interest and context.
Anyway, I've gone over the paths a few times and the ones for the visuals is on the gimbal and the audio commentary is the same cam chest mounted but no gimbal.
Looking at the footage side by side, trail riding along gravel paths with bumps in you need to look closely to see the differences. When I go over bumps, the gimbal assembly bumps up and down if I forget to wear a second chest strap at the top of the chest mount to stop this. (cheap chest mount!) This is visible in a slight up and down bounce - but only if you know where to look, and what you are looking for! I have the stabilisation on with the camera too.
Without the gimbal it's still smooth, but the gimbal removes the side to side swaying rhythm that's visible when you are pedalling hard.
I've had to do some retakes as I've acidentally had the camera on HDR mode with doesn't have the stabilisation - even gimbal mounted footage.
I'm looking for some movement to suggest bike riding over trails, but not movement that's going to be irritating. What works best for my purpose is gimbal with side to side panning and the camera in 4K with d-cinelike and rocksteady on. I can do stuff with the footage in post better with d-cinelike, plus it fits in better with the
Mavic 2 d-cinelike footage.
I've tried it on dull days, and to be honest it works fine. I don't tend to use dull day footage because dappled sunlight through trees is more visually appealing!
I would say that the Osmo is a bit on the heavy side, so whatever gimbal you have you may well need to add some couterweights to balance it. My gimbal will turn off if it's not balanced. I did have another gimbal that said it didn't need to be balanced as it had "Powerful motors" that simply got hot and died.