"Spend" refers to any ability that allows a player to choose to pay gold or move gold from his or her gold pool. This includes paying costs and moving gold for bestow. (See Spending Gold.)
You can choose any card in play, even if it does not have the bestow keyword or any other interaction with gold in its text. You can also choose a card with bestow that already has gold equal to or higher than its X value. You cannot add any gold to your gold pool with Casterly Rock.