For the digital codes, the choice is between GOG and Steam, not *both.*
Either digital retailer option allows you to download the Myst 25 games from your chosen digital retailer (GOG or Steam), and the downloads will be able to be played on either Mac or PC, depending on the operating system you're using when you log in to your account. You could play on both if you have both (but not at the same time).
The digital games will be available on GOG first, then will appear on Steam later in the year (estimated 2018).
When they arrive on the digital storefronts, they will also be available for the general public to purchase.