I bought Witcher 3 through G2A but the platform is GOG, im confused because my game is installing through google downloads ;-;? is there not a download/launcher ?

That image shows you are downloading the GOTY version, from GOG content delivery network, that includes Galaxy in the installer -- denoted by the (g) in the file names. That means that (unless you already have Galaxy) it will be installed when you run the installer package.

Your biggest worry should be is the key you purchased from G2A a legitimate key? G2A is notorious for selling illegal game keys.