backup downloaded through Galaxy is a 'classic' installer - game setup without additional Galaxy Client installer

'galaxy' installer is a game setup that has bundled GOG Galaxy install, that will try to install Galaxy by default if there is none on your PC. Checkbox whether you want it to install or not can be found in Options page at the beginning of the installation (together with install path etc). That choice will be remembered for future installations.

When downloading from browser, you can switch between downloading classic or galaxy installers on game page in your library (by clicking 'More' and selecting corresponding option'). That choice is not being saved atm and has to be repeated each time you download the game.