Can gold be transmuted in the Fullmetal Alchemist continuity?
Yes, transmuting gold is possible (at least in the manga). The reason why it's forbidden is stated later in the series: It's forbidden so nobody can destroy the economy by making the gold worthless.

The mass is also conserved, I see no problem there. A ton of coal makes a ton of gold. Equal mass :).

Note: In the first adaption, Ed did not transmutate gold but instead coated the coal in gold. Either it's not possible in the 2003 anime or he didn't want to violate the law. (Thanks to キルア)