Can I use an action gear from my hand?

Yes, you can play an Action gear from your hand, but its timing depends on whether it has the Action keyword or is being played normally.

Here is the breakdown based on the rules:

• Normal Timing: Unless a gear has specific keywords allowing otherwise, it must be played during your Main Phase while the game is in an "Open State" (meaning there is nothing else on the chain and it is not during a showdown). Gear are played to your base by default (Rule 149.2).
• The Action Keyword: If a gear has the Action keyword, Rule 806.1.c.1 states it allows the card to be played during showdowns on any player's turn. Note that Action is a permissive keyword—it grants permission to play it at these additional times, but it does not prevent you from playing it during your own Main Phase under standard conditions (Rule 806.2).
• Important Distinction (Equip): If you are trying to attach a gear, be aware of how the Equip ability works. If the gear has an Equip ability, that is an activated ability, not the act of "playing a card" (FAQ 9148). Standard Equip abilities can only be used during your own turn in an Open State (Rule 150.2). If the gear has the Quick-Draw keyword, it inherently has Reaction (Rule 819.1.b), which allows it to be played and attached using reaction timing.

In summary: You can always play an Action gear during your own turn in an Open State. If it specifically has the Action keyword, you also have the permission to play it during a showdown on any player's turn.