I have a 2019 Gain M20i, US version (20mph cut-off speed). I've ridden it just over 9000 miles on just over 500 rides. I would describe the loss of battery capacity as negligible. On identical rides on similar days with the same power levels, I used to go 12 miles before I went under 75% capacity (white to green), now it's 10.5 to 11 miles. So perhaps 10% loss of capacity.
Orbea's technical specs and manuals (which you can find online) consistently note an expected reduction in capacity of 20% after 300 cycles, and 40% after 800. Along with an estimated total life span (service use) of 5 years.
In my case, I have tried to be meticulous in my charging habits. Most notably I charge after every ride, even if I've only used 20% of the capacity. So I've plugged it in 500 times. I don't know if that constitutes a 'charge cycle', as the definition varies. Across 180-200 rides per year, I use more than half of the capacity (green to amber) maybe 50 rides, and more than 75% (amber to red) 20-30 times. And I've only gone into 'limp mode' (flashing red) 10 times in the two and half years I've owned the bike. So I've only rarely deeply discharged the battery.
Overall, I'm very happy with the capacity performance over time and mileage. Most notably, for my most common distances (<25 miles) there is no worry that I'll go into the red, and have any battery anxiety. For longer rides, I have a supplemental battery - my rule of thumb is anything over 40 miles. If the capacity dropped to less than 70% I would get a replacement battery. I view that as 'standard maintenance' for an e-bike. Is the dealer charge a rip-off? Well probably, but even if I only get 800 rides out of my battery that's a dollar a ride, less than the depreciation of the bike.
**Note - it has occurred to me that I'm
trusting that the controller is giving me an accurate representation of the remaining capacity. Meaning, when the software says that I've just gone from white to green (below 75% capacity), do I really have 75% left? In the few times most recently that I've crossed below 25% (amber to red), it goes pretty quickly to slow-flash-red (below 15%).