The best way I found to do it is to use Free Download Manager and continue downloading until it stops (ie the download link expires). Restart the download by starting a fresh download from your GOG account. When it opens in FDM stop it and right click on it and then click download properties. Copy whatever is in the "URL:" section and paste it into the URL: section of your 50-75% completed download that you started earlier. Click start at the top of your FDM toolbar and hopefully it should resume where you left off...
All the best.
I have no problems using my browser to download large files, if they`d work (this is the first time I`ve had problems doing it). I`ve tried downloading my newly bought Arcanum twice, both of which failed randomly at some point. I just left it downloading and would check back regularly to see how it was doing. The first one died somewhere around 75% and the second try I`m not sure. I left it going and went to watch a movie. Once I got back it was dead.
Thus, I downloaded Air and the downloader, only to find out it`s seemingly useless. I can`t find any way to actually download my games with it. It`s just blank and if I click a game in my account it only gives me the option of downloading with my browser again.
I`m now trying it a third time. I`m getting around 650 kbés, so it should take about a half hour. Good start, but the other tries started well too.