I've bought games and installed them via gog galaxy on one pc. How do install them to another pc without having to purchase them again?

Install Galaxy on the other pc and install them just like you did on the first? You shouldn't need to purchase them again, though downloading is something you'll probably need.
You can of course copy the files over and use Galaxy's import feature, which should result in downloading less data.

Edit: If you are going to be installing the games on more than one PC, it might be better to download the installers and run those on each PC. Would probably save a bit of time in the long run.