Weird part of 2024 True Polymorph is you can't speak.
...The target is limited in the actions it can perform by the anatomy of its new form, and it can’t speak or cast spells.
While you could speak in 2014 if the new form is verbal.
...The creature is limited in the actions it can perform by the nature of its new form, and it can’t speak, cast spells, or take any other action that requires hands or speech, unless its new form is capable of such actions.
Depending on Oxford comma usage, this might even say you can cast spells if the creature can, and Gold Dragons are spellcasters, at least in 2024 and maybe Fizban. Of course, the caster has lost access to their spells from the part about ...The target’s game statistics are replaced by the stat block of the new form...
That doesn't mean what you think it means in this case. You can't cast True Polymorph on a Gold Dragon as it will reflexively shapeshift out of it. Nothing says you can't cast True Polymorph on a non-shapeshifter and turn them into a shapeshifter, such as a Gold Dragon
