I feel that most bad cards shouldn't be directly errata'd, because new cards can indirectly buff them to make them worthwhile again, or players might uncover niche strategies that utilize those cards in some way. However, there are a handful of cards I do consider bad enough to be broken outright. Nothing short of an errata can fix them, and their current iteration is basically unplayable: they might as well not exist because absolutely no one ever plays them. Gate to Hell has always been one of those cards. I consider it a shame, since I don't think it would actually take that much work to give the card a strategic function while keeping it unique.
I don't think the card should have its cost reduced, nor be made a quick action to cast. Instead, it should become the first late-game spawnpoint. Add an innate Channeling of 3 to the card. Then replace its current card text with this:
"All demon creatures gain Melee +1. Gate to Hell enters play with a "Closed" token. Once per game, as a full action, you may pay 12 mana to flip the token to the "Opened" side, and make the above attack against each non-flying creature in the arena. Once opened, Gate to Hell gains the Spawnpoint trait and may summon an unlimited number of demon creatures during the Deployment Phase."
Now all of the separate parts of the card (the demon melee buff, the arena-wide attack, and the summoning capability) suddenly synergize with each other. You can lay down the card early, protect it during the mid-game, and then suddenly weaken all of your opponent's creatures right before summoning a swarm of your own. This also doesn't make the card overpowered. It hurts the early-game economy of its mage, is vulnerable to being destroyed before it becomes useful, and isn't particularly useful outside of Warlock swarm builds.