VFansss: That would lighten the traffic on the GoG CDN, but would be complicated to dedicate resources to its developing.
I dunno. I am unsure what exactly it would take to implement such system, but for some reason Humble Bundle has come up with some kind of p2p solution, and I am pretty sure they have less resources and staff than GOG. The DRM-free installers (both for PC and Android games) have links both to direct web downloads, as well as bittorrent links where you get to use any bittorrent client to download your games. Not sure how exactly they check that you are a legit downloader and not "tapping in" to content you shouldn't be downloading, or do they just trust you not having access to torrent links to games you haven't purchased? Someone who is familiar with bittorrent innards probably knows...
And as said, many other services and apps seem to have implemented it too, like the Android freeware car navigation program I mentioned, or Windows 10, or the Battle.net client (at least in the past). If bandwidth is cheap, why then are the others still doing it? Plus, it wouldn't necessarily have to be officially from GOG, could be third-party tool too, as long as it allowed only legit customers to "tap in". I certainly wouldn't want to serve any filthy pirates, they never shower yech.
Anyway, the most important reasons I'd like to see
optional p2p protocol used is that it would probably better guarantee high download speeds also for those freaks with 100Mbps - 1Gbps internet lines, and also be an option for those who currently complain they are getting too slow download speeds. I personally am fine and pretty much always get full download speed on my measly 10Mbps internet line, albeit occasionally I've seen some web downloads come only at 400kbps or so (but if I download several files at the same time, it doesn't matter as together they max out my internet connection anyway).
And if it helps the GOG CDN servers as well, all the better. I guess I am also fearing that GOG considers third-party mass-downloader tools like gogrepo and lgogdownloader "unwelcome" because of the extra load they can cause to GOG servers, and possibly try to somehow restrict or even prevent their usage. For instance:
https://www.gog.com/forum/general/gogrepopy_python_script_for_regularly_backing_up_your_purchased_gog_collection_for_full_offline_e/post1303 (albeit that is not really related to download speeds, but at the rate of which GOG is serving incoming http requests, I presume)
VFansss: Also, the force of GoG is its simplicity : buy -> download -> play. Even without internet, without DRM, without anything that isn't strictly required.
I keep saying it would have to be optional anyway, so it is not like a normal user would even see it. You could enable it separately, or even use a completely different tool/client for it.
Until Galaxy arrived, I presume Steam was the place where people looking for simplicity went by default. Installing and playing GOG games was always a bit more involved, having to separately download several installer parts, install the game manually, then if an update was released for the game, many times it meant downloading and re-installing the whole game (no auto-update either).
But yeah, the lack of DRM is also certain kind of "simplicity". Just a different kind of simplicity. :)
VFansss: And, finally, bandwidth isn't so expensive: they already pay for their server and I don't think they "pay" based on how much bandwidth they used.
Really? I have no idea how the pricing goes with many CDN server providers and such, but I presume people downloading more does somehow increase GOG's costs. I did at some point get involved on e.g. how Microsoft charges for usage of their Azure cloud services, I am pretty sure there were at least different price points to how much you were expecting traffic and downloads from "your" cloud servers. I don't recall if there were options where you were automatically charged more if there were more users and data transfers, or would excess users just be blocked away.