just look down and you'll find the offline installers if that's what you mean

Look up the game in the library, ignore "Install Game with GOG Galaxy", and expand the "Download Offline Backup Game Installers" section instead.

Download everything into one place on your HDD/SSD, install and voilà.

Hover your mouse pointer over your [Account]
-> [Games]
-> choose the game you wish to download/install
-> click on its thumbnail
-> if there are no offline installers visible, you have to first click on the [Download Offline Backup Game Installers] to make them visible
-> now you see at least one .exe-file (depending on the game, there may also be several additional .bin-files)
-> click on that .exe-file to start the download (if there are .bin-files, you have to download all of those, either)
-> once you have downloaded all the files, go to your download folder and double-left-click on the .exe-file
-> follow the instructions, choose a destination for your installation(s)
-> after the installation process is finished, you can either start the game directly from the pop-up window, or from the symbol on the desktop or from within the installation folder of the game.

Have fun!

Are you using the website in your web browser or using Galaxy to get your games?

If the first, then follow the instructions from BreOI72.

If the second, then Galaxy has a menu option called Extras, where you can browse to the Offline Installer versions of your owned GOG games.

U R wellcum.