You must show your chosen card to your opponent to verify that it is a Spell Card.

You can select a Field Spell Card but you must Set it in your Field Zone.

If you get a Spell Card with “Ancient Gear Drill” and then return it to your hand with “Giant Trunade”, restrictions no longer apply so you can use it as normal.

“You cannot use that Spell Card this turn” means that you cannot activate it, and you cannot use it to pay a cost for Summoning “Anteatereatingant” or for the effect of a card like “Emergency Provisions” or “Elemental Hero Tempest”.The latest errata of "Ancient Gear Drill" only states that the Spell Card cannot be activated, not that it cannot be used.

Ojamagic: If you select "Ojamagic" for the effect of "Ancient Gear Drill", and it is destroyed that same turn, you cannot activate "Ojamagic's" effect.