I'm trying not to confuse myself here,but if I were to install the installer on the website,then just download the game through GoG galaxy,can I still drag and drop the installer onto a flash drive and use it later? Do I have to bring the whole folder on the flash drive or just the installer? Sorry if I sound confusing.

If you have the installer file, you can install the game directly from it - you don't need to use Galaxy. Even if you do use Galaxy to install, if you select the option to backup the installer, then once you have the installer file, you can move it to another computer, as you have described. Basically, all you need to install a game is the installer file(s).

EDIT: Curse you JMich...

If you mean backup installer from More -> Backup & Extras then yes, that one will be enough to install the game in the future.

Thanks everyone for the quick replies.love this community :)

If you download the game directly through your browser; with the GOG downloader or by using Galaxy's "backup" function, you'll have a stand-alone, offline installer that you can store and use wherever you like.
Using Galaxy's "install" option will do exactly what it says: unpack the game directly onto your system, which makes transporting it much harder, if not impossible.

For some further info, I'd like to refer (once again) to this post I've made some time ago.

EDIT: Never mind...