GOG Installers are based on the InnoSetup and are offered in 2 versions currently
The main difference between them are in size, as the Normal includes Galaxy client (With the option not to install it), whereas the latter one doesn't. The installers can be downloaded from your Account page at the GOG site or through Extras in a selected game at the Galaxy client, which will download Classic version only.
Another thing worth to mention is that regardless of what installer version you are using, it will save the options you have made in the Options tab before installing the game and later on load it regardless of the game.
Galaxy Cloud is a feature of the GOG Galaxy client that allows save game data to synced to the Cloud. This feature is made available to the public in version 1.2. List of games that support cloud synching can be filtered on the store or very first list of Galaxy Cloud games can be seen on this page.
Although Universal Cloud Saves in Galaxy Client needs to be implemented by the developer, some games Planescape: Torment, Heroes of Might & Magic® III, or Vampire®: the Masquerade - Bloodlines received the first time save syncing in the cloud .
While the Galaxy client for Linux is still in development, an unofficial client for GOG has been created using Python, while using LGOGDownloader to download any game through the old API used by the GOG Downloader. The client features the built-in launcher for each game, integration with Wine, DOSBox and SCUMMVM and configure it for the specific game, applying community patches and more. It is currently in a Prototype state, but usable and installation is very simple by just running the install.sh script, make sure to download the required dependencies to get it to work properly (Besides Wine).
Sales on GOG.com happen in regular basis, the most recent one called Weekly Sale, which starts in each Monday at 11 AM GMT, additional games are added to it in each Friday at the same hour the sale starts. Big sales usually happen a week or two before Steam's happens and adds themed bundles on sale, along with single games in every day. The best moment to redeem Steam games to GOG.com via GOG Connect is when the sale is coming to an end.