1. Are the games Windows 7 compatible?
2. Are the games Windows 95/98/2000/ME compatible?
3. Are there plans to make them Mac compatible?
4. Is it possible to copy the game files to a disc so you can play the game on an older computer?

1. Most of the time.
2. Probably.
3. No. You can use Crossover or Parallels to play it though.
4. Probably not.

2. I doubt the installer runs on Me (it doesn't on 98), the games themselves probably do. 2000 probably works.
4. For some, yes, others, no.

1. Some are, there's a topic on that here: http://www.gog.com/en/forum/general/gog_com_games_working_on_windows_7_64bit_o
2. Only Windows XP and Vista are currently officially supported by GOG, so it's unlikely that their installer will work on older systems such as 95/98/ME
3. Probably not.
4. As others say, that'll probably depend on the game, but it's not officially supported.