Then they are obviously not the native mpeg4 or H.264 format videos. More likely they are DivX, Xvid, or other format that your QuickTime doesn't have plug-in codecs for. See http://www.gnutellaforums.com/292880-post4.html & I highly recommend Perian. Though I also have most of the others installed also.

You may need to re-encode the videos to get them onto iTunes, but wait until after you install QT plug ins. Also, don't forget QuickTime Preferences -> Update -> Install. Here's the direct link for you Apple - QuickTime - QuickTime Components
Another thing to be aware of with QuickTime is QuickTime Preferences -> Advanced -> MIME settings & making sure the appropriate video & audio codecs are enabled.

Edit: I forgot there are some free video conversion tools for MacOSX video, here's one Apple - Downloads - iPod + iTunes - iSquint So if after the above you still can't add or play the videos within iTunes, then you may need to convert the video to make it compatible. Conversions should only be done as a last resort because each time a video is converted it loses quality. And all videos you find on the gnutella network have been previously converted down in size, often dramatically down in size so their quality is nowhere as good as the original.
It's not only the video file size that counts with quality; it's the video viewing dimensions, conversion from one file type format to another, quality settings, quality of the conversion tools/codecs used, ... each file type format being a "lossy" format in itself meaning it throws away information in the process of saving into that format, regardless of whether quality settings are set to maximum.