Infinity Gauntlet's digital unavailability follows the disappearance of several Marvel-branded games earlier this month, as well as Capcom's pulling of Marvel vs. Capcom 2 and Ultimate Marvel vs. Capcom 3 in December. It's possible that Disney's acquisition of Marvel in 2009 is resulting in lapsed licensing agreements, but no explanation has been given as to why these games have been removed from console storefronts. In contrast, the more recent LEGO Marvel Super Heroes is still available in both stores.
Of course, you can still hunt down retail copies of any Marvel games you were still considering purchasing. Considering warnings for some of these delistings were scattered across social media accounts, retail copies might be the easiest option to keep track of anyway.