The gear card 'handgun' says I can discard one success and exhaust a card to deal damage. Do I have to activate an attack card first to use this or can I carry out any action activation? i.e can I activate Aid, use the hand gun and exhaust explore to do so?

I have the same question regarding the Hand Axe.

Also an additional question: When you "discard 1 success token" per the Hand Axe's instructions, can you remove the success token from ANY action card that you have that contains success tokens?

Thank you for any help.

For both cards, same answer: Any action will satisfy the first condition of the gear card, not just an attack.

However, let me just point out: The action card you then choose to exhaust to activate the gear card's effect cannot be the action card with the prepare/ready symbol (which one this is depends on which hero you are playing). Hero action cards with the prepare icon can only be exhausted if that card is activated as the chosen action for the turn, and not by any other effects.

And yes, when discarding a success token you can discard one from any of your action cards.