I don't know what the specific problem was in your download, but the gog support links are here: http://www.gog.com/en/support/planescape_torment
Planescape was the 1st game I downloaded as well, and I thought I was doing it wrong (because every time it finished downloading to my desktop, the file wouldn't open; it turns out that my firewall was stopping it from doing opening, so I turned off my internet connection, then turned off my firewall, and then the game file opened and thus installed perfectly).
So what I did was perform the steps in Number 4) on this page: http://www.gog.com/en/support/planescape_torment/_i_general_download_problem_troubleshooting_steps_i_
My advice to you is to download one game at a time. Like 1st download Planscape, then once that it was done, only then start downloading BG2. Perhaps it may even be something silly like your internet service provider messing up via a power outage or something like that.
Hope it works out for you! Those two and BG1 are the best games ever!