Everything is pretty much the same for GOG, except the difference with the install path, and disabling cloud support (to do so globally, click on the GOG.com button and select Settings, and then Features; just for D:OS 2, with the game selected, click the More button and select Settings, then near the bottom unselect the Cloud saves option).
Yes, it is. The data for the Classic version can be removed, but with the next game update, Galaxy would want to re-download it. There are a couple of ways to get around that, though.
The first may not be feasible if you want to continue using the cloud save feature. If you have another drive with lots of free space, where the size isn't a problem, you can simply copy the folder there and 'import' it in Galaxy to change the install location (took a quick look, and Galaxy doesn't appear to support moving install folders directly). Then you can delete everything but the '..\Divinity Original Sin 2\DefEd' folder on the current drive, and start the game directly from the executable. If Steam is running the game will connect when starting from the executable to make Steamworks functions available, but this doesn't appear to be the case for Galaxy. To sync saves you would need to restart the game through Galaxy after playing and quitting the instance from the executable. After Galaxy updates the install on the larger drive, you could then manually copy the DefEd folder to the current location again to update that folder.
Alternately, you can move the original release data folder to a USB flash drive, if available, or a different drive, and then use a symbolic link
to make the files and folders appear to be at the expected location for Galaxy, so it doesn't redownload them if you verify files or the game updates (in the game Settings in Galaxy, you can disable automatic updates, so if you use a flash drive you don't have to keep it plugged in).
In the 'Divinity Original Sin 2' folder, 'Classic' and 'Data' can both be moved, as well as 'bin' since you wouldn't be using the launcher; most of the size is the 'Data' folder, though.