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.