If you pray until your god curses your inventory, this will curse your gold.
That is, if you equip your gold pieces in your weapon slot you will be unable to unequip.
I also have the impression that merchants throwing cursed gold deal less damage to
undead. If the PC picks up gold while also carrying gold, the stacks are merged, even if
they have different item status.
It seems gold was supposed not to have an item status? In any case, this behavior is
weird and probably a bug.
It also works if you dip gold into unholy water, then equip it. However you need to have 19 gp maximum, otherwise nothing seems to get cursed. If you curse 19 gp, then equip one, bless it, remove, you again have 19 gp of cursed gold.