These characters are master manipulators.... they play their parts and use whatever comes their way to their advantage. People believe what they want to believe, thus, they are well able to deceive those around them.
The gulls are "gullible," easily led to lend their belief to the tricks and plots of the conmen. The play itself is obsessed from the Prologue onward with the idea of what Coleridge would call the "willing suspension of disbelief," except that the gulls do not really start with much or any disbelief, and this is reality for them, not a story in which they believe the premises of a story in order to see what the author does with it. As Jonson’s audience, we know that the stories (and the whole play) are not real, so we are not gulled.
Gold is the result of successful alchemy, though the goal remains aspirational. It plays a large part in the play as the motivation for just about everything that happens. The gulls are all greedy for gold in order to achieve their dreams, and they are therefore greedy for the Philosopher's Stone. The conmen, inversely, are greedy for the gold they make by tricking the gulls into believing that they will eventually be rich.
Face's epilogue considers the fact that a theater audience similarly has handed over gold in order to be knowingly tricked with a false story on stage.
