I can't seem to download XIII at all while using the GOG Downloader. It will start off with a few MBs (e.g. 6.5mb) and increasing then suddenly it will jump to around 1.8gb-ish. This would have been the best if it actually downloaded all the bytes in-between the jump but after fully completing the download, it would have some sort of checksum error. Which is understandably because of the cut download.

Every time I try to download it, this happens. Is this thing happening to others or is it isolated to my case?

Hmm, weird, I bought it yesterday (12/31 happy new year :P) and it downloaded fine. Try running the downloader before downloading it as an admin.

I think it's got something to do with my shitty internet, but still, I thought the main purpose of the downloader is to be able to retrieve the installer no matter what.