Source: Valve
Steam made a recent update that will make it easier to help friends complete the games in a series. Valve now lets you purchase the Complete the Set bundles as gifts, including the discounting feature. As PCGamer pointed out, when you buy a bundle for someone else who already has some of the games in the bundle, you will pay less overall. So, if you're thinking of a gift for a gamer friend, this might be a good option to explore.