You paid a lot of extra money for this feature, you should be aware of it.
The new battery has to be custom made, then ensues a royal PITA to install it inside the tubes of the bike frame. You wanted an electric bicycle that no one would notice it was electric, this is the price you pay.
Usually bike shops do not have electricians and spot welders for this kind of work, even a regular ebike shop. Battery might be available as modules from the company, pre-welded, but all wires must be snaked thru the frame, likely multiple battery modules. I have seen a detailed breakdown of a similar install, a full days work is not an unreasonable estimate, especially since they don't likely do very many, and virtually no commercial ebikes use such a system, for good reason.
Most e-bike batteries can be replaced in less than a minute.
Did they tell you about the available range extender that looks just like a water bottle, that you can install yourself? Which could be used instead of the in-frame battery?
For the record, there is only one battery to be removed from the downtube of a 2020 Gain. There are wires to be disconnected at 3 different locations, but only one battery to replace. Panels in the carbon fiber frame are unscrewed for access to the wires, which stay clean and dry inside the frame.
If you can follow a YouTube 2 min. instructional video, the change seems straightforward.
The video title is: Hello Ebikemotion Battery Removal