So, yesterday i wrote to the support but didn't get any reply yet. I was having problems with the download manager, restarting my downloads all over after downloading the 5~7% of the game(Colin Mc Rae 2005, around 3 gigs), although my previous download, FlatOut(900 MB) came down like oil. Anyway i've managed to download CMR through the browser download without any problem in a short time.
Now i'm trying to download Unreal Tournament 2004, 2 gigs: already tried the download manager and gives the same issue as the previous download, so i decided to go for the browser download again: another problem, download speeds go high to almost reach my connection cap for a few seconds to then go down to 0 and stick there for minutes(while i can browse web pages with no problems). Is it a known issue? I can't leave the pc on 24/7.
Thank you.

The servers themselves are pretty fast (Just downloading BrokenSword3 for testing: 1.4-1.6 MByte/s out of a possible 2), so it's probably more of a connection problem (assuming that your provider isn't throttling you).
When you download through Firefox, right click on the download and chose "Copy download link", your clipboard should then contain something like this (don't post it, GoG probably wouldn't want this url to be public):
http://bitcast-a.v1.fra1.bitgravity.com/gog/secure/program.exe?e=secret&h=secret&ext=.exe
the important part is everything before /gog/ , in my case
bitcast-a.v1.fra1.bitgravity.com
This is where your download is actually coming from and the speed to this point is usually the bottleneck. Try to paste it here to find out where the server is. Mine is about 200km away, maybe they don't have a mirror in your neighborhood.
Edit:
You should also do a traceroute to that server:
Open a console window and enter
tracert bitcast-a.v1.fra1.bitgravity.com
(of course replacing the server url with the one you got)
You should get a table listing the time to reach each hop and be able to find the bottleneck there.

This is the traceroute:
1 * 3309 ms 2097 ms 192.168.100.1
2 * 55 ms 2989 ms *.20-80-b.business.telecomitalia.it [80.20.193.*]
3 * * * Richiesta scaduta.
4 * * * Richiesta scaduta.
5 * * 86 ms mil30-ibs-resid-3.mil.seabone.net [195.22.192.185]
6 713 ms 61 ms 168 ms mil26-mil30-racc1.mil.seabone.net [195.22.205.246]
7 162 ms 111 ms 161 ms fra18-mil26-racc2.fra.seabone.net [195.22.211.214]
8 * * 76 ms fra7-fra1-racc1.fra.seabone.net [195.22.211.217]
9 113 ms 2987 ms 922 ms teleglobe-2-ca-fra7.fra.seabone.net [195.22.211.122]
10 * 3132 ms 182 ms ix-2-0-0.core2.FR1-Frankfurt.as6453.net [80.231.65.18]
11 1857 ms * * bitcast-a.bitgravity.com [208.67.237.237]
12 * * * Richiesta scaduta.
13 * 98 ms 2995 ms bitcast-a.bitgravity.com [208.67.237.237]
Can't tell much from it, except, maybe, that the server is located in France.

It's a bit hard to read with this formatting (plus, I don't speak Italian), but it looks like your connection is failing all over the place with usually 3 attempts before you get ANY response. The strange route it takes looks like your provider is avoiding another provider. You probably will have to ask them for customer support.

Dunno, now the speed is stable, but at less than 60k/s. absurd.
That doesn't depend on my connection because if i try downloading something else from another site, it goes almost full speed.

packet loss could be an issue coming from your service provider, also they may not understand the routing. you might want to phone your ISP and ask if there are any issues in the area or if they could look at your connection.

Or on weekdays either, I've sent multiple items to support and never heard anything of it, over half a week past and eventually I found the answer through the help of another user but..

It is not possible to use a download manager right? At least i could be able to resume the dl, and it wouldn't fuck up the file telling me it's completed after downloading the 15% of it, like Firefox does after some connection problems.
I tried with Free Download Manager, putting my login information, but it does not work.

You can either use the official download tool or something like FDM. However, there's one thing you have to be careful about when using FDM: You won't be able to resume... or rather, not directly, because the url expires after a certain time:
If you want to resume, you have to initiate the download again on the gog page, but don't add it to FDM, just copy the url. Then, open the properties of the download you want to resume and replace the old url with the one you copied from the new download. Then, resume. Repeat every time you want to resume again.

I have no problems with download managers.
In fact I just had one problem, when I was downloading from my school with 16 connections at a 10MB/sec speed, the server banned me, got a 403 in the middle of the download :D

You probably didn't resume.

What do you mean? IDM told me "response from the the server - 403" at approx. 70%, when I tried to resume it told me to go away like "i dun't liek u, forbidden lawl" :>
But here with a download speed less fast (?), I don't have any problems.

I meant the first part "no problems with download managers"
The 403 was probably exactly that: your session had expired.

I finished downloading the game, after ups and down and hours of download and.. game is corrupted. kinda frustrating.
I'd use the downloader, but why has it to reset the download if there are connection problems? Can't it just pause and resume?