Typically you get a little more damage for the same cost than you do threat clearing. You might get, say, five damage for the same price you'd pay for four threat removed. So you could evaluate the drawback of increased minion HP on Gatekeeper as being equivalent to a reduction in the amount of threat it removes. In total, this card may be worth something like 2.4 threat removed for the cost of the card, which is still pretty good (much better than that there For Justice! certainly).

It is, however a very conditional card: you need there to be a minion, you need to be able to cleanly remove that minion with its newly increased HP, and the threat removal can only go towards the main scheme, so it doesn't help with side schemes and there's the usual issue of "is there going to be that much threat on the main" in solo and "do I even care about main scheme threat / is someone else better at clearing it" in multiplayer.

As a bonus feature, this isn't a Thwart, so it doesn't care whether you're Confused.

Might be best in Cyclops (turns on Optic Blast) or Shadowcat (turns on Acute Control, though that doubles up on the awkwardness about thwarting the main). If only Outta My Way! was a better card!

Others have pointed out this card's inherent clunkiness and I'm certainly not going to argue against that. It's definitely a weird card, but it has some some surprising flexibility, too. It's an interrupt, and not specifically a hero or AE interrupt, so it has some extra value with heroes that like to flip a lot and get allies on the table. X-Men and Guardians both have cards that encourage ally swarms/mini-voltrons even in aggression and Gatekeeper can be nice for letting your beefed up allies contribute to thwarting while keeping the table clear of minions. I know some earlier reviews compared this to Chase Them Down but I think there's a better comparison to be made with War Room. War Room has more flexible scheme targets, but Gatekeeper can be triggered by any hero or ally and it removes a much higher threat value (albeit only from the main scheme).

Others mentioned that it potentially has more value in multiplayer and I think this is where the card really shines. It's a great "side board" card. A dedicated thwarter will make this card obsolete, but 5+ years into the game, not every justice player wants to be a dedicated thwarter. A copy of this card in your deck can contribute to keeping threat low with a wider variety of other decks in multiplayer. Hard to recommend building a deck around this card, but it can be a decent flex option.