Does Forge of the Future kill itself as a cost? Or do I need to play a card that kills the Gear?

Forge of the Future kills itself as a cost—you do not need another card to kill it for you.

According to the card's ability text, "Kill this: Recycle up to 4 cards...", the instruction to kill the card precedes a colon (:). In Riftbound, any text appearing before a colon in an ability is a cost you must pay to activate the ability (Rule 150.1).

Here is how the process works, based on Rule 150.2 and RiftJudge FAQs (#5082, #5382, #9207):

1. Activation: During your Main Phase in a neutral, open state (when no chain is active), you announce that you are activating the ability of Forge of the Future.
2. Paying the Cost: You pay the cost by choosing to move Forge of the Future from your board to your trash. Because this is a cost, you do not need another card effect to destroy it.
3. Ability on Chain: Once the cost is paid, the ability "Recycle up to 4 cards..." is placed on the chain as a pending item.
4. Resolution: Players now have the opportunity to react to this ability (as it is now on the chain). If both players pass, the ability resolves, and you carry out the recycle effect.

Crucial Nuance: Because killing the card is a cost to activate the ability, if Forge of the Future is killed by an opponent's card effect (such as Acceptable Losses) or a game rule before you have chosen to activate it, you cannot then activate its recycle ability. This is because you are not paying the cost to activate it; it is simply being destroyed by an outside source (RiftJudge FAQ #9207).