The Game I'm downloading is Divinity Original Sin 2, which is a 27GB download package. The Download location is on an external hard-disk.

I began downloading it after midnight to avoid exhausting my peak time data quota, and unfortunately my download speed is incredibly slow - it's been 8 hours so far since the download started. This was not my first attempt to download it - i did so earlier a few days ago, and was able to get 4GB downloaded. When I began downloading last night I misundertood and 'Uninstalled' Divinity Original Sin 2, but when i resumed the installation it checked my local files and discovered DOS2 on my Hard-Disk.

If you refer to the images attached you can see that the Download Progress Bar next to the games title states that I have downloaded 15GB so far, whereas when you look at the bottom right hand side of the GOG Galaxy app (as displayed in the other image) the progress bar and the data downloaded is stated to be as 12GB.
I am afraid that this discrepancy means that some sort of data is missing - And i'd like to avoid that. My internet is mind mindbogglingly slow and my internet quota for the month is reaching around 15GB.
Please advise me on what I might do!
Thank you!

Use download manager to get offline installers.
After installing the game, detect it in GOG Galaxy.

Do you think I need to restart my current download if I use the Download manager?
Also thanks!

Files for offline installer and GOG Galaxy are different.
If you decide to get offline installer, you have to download them from scratch.

lgogdownloader [1] get GOG files via Galaxy API, but I do not know if it can resume files from where you stopped.

I use gogrepo.py and wget, both can only download offline installers.

[1] https://www.gog.com/forum/general/lgogdownloader_gogdownloader_for_linux

Silly me - i realize my mistake. The number given next to the games title refers to the GB that is yet to download. I will be using this downloader you speak of next time i download a game. Thank you for helping me!

I think that galaxy also downloads the archived files. Because from what I remember it says something like 4GB to download, 6GB to install.

GOG staff have said that Galaxy grab different files to install games.
Galaxy could also be used to download offline installers, but it is another case.

I don't doubt that it's different, but still I think they have some compression.