Anything you shared to the actual gallery will be available for you to re-add to your library. If it's just in your library it will not transfer to the new computer. So if there's any builds or sims you want to take with you to the new computer either upload them to the gallery or back up your tray file folder to a jump drive or something and simply replace the new tray file that pops up when you install the game with the old one. You can also do this with save files if you want to keep a current save file going on the new computer. Just back up your saves file and replace the new one with the old one.