Once you have finished placing and paying for your order, the game will be added to your GOG account and can then be downloaded - you can either use GOG GALAXY 2.0 or your browser. If you want to learn more about how to download your purchased game, you can find more information here.
If you can't see the game in your account's library, your order might still be processing; unfortunately, this can sometimes take up to two hours. Should the game still be unavailable after two hours, please try to refresh your account manually by clicking here.
If that didn't help, please also read this support article or contact our support team so we can investigate the issue.