If you cast a card “without paying its mana cost,” you can't choose to cast it for any alternative costs, such as bestow costs. You can, however, pay additional costs. If the card has any mandatory additional costs, such as the commander tax, those must be paid to cast the card.

Geode Golem's triggered ability triggers even if your commander isn't in the command zone at the time Geode Golem deals combat damage to a player. If your commander finds its way there before the trigger resolves, you can cast it.

If you wish to cast your commander without paying its mana cost, you must cast it while the triggered ability of Geode Golem is resolving. You can't cast for free it later in the turn. It may be cast at a time you normally wouldn't be able to cast a creature spell, but other restrictions (such as “Cast this card only from your graveyard”) are enforced.

If a spell has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

Geode Golem's triggered ability triggers even if it's dealt lethal damage at the same time it deals combat damage to a player.