If you buy the games as a gift for yourself (by checking the box saying it's a gift and then giving your own e-mail address), then you'll get a gift code by e-mail that you can either redeem yourself whenever you want to or give to someone else. These codes will also be listed in your account under the "gifts" tab of "My games".

But as soon as the game shows up on your GOG shelf because you redeemed it or bought it directly, you can't gift it anymore. If you made an error buying it though, the GOG Support might be able to help you out, as long as you haven't downloaded the game yet and immediately tell them about it.