Do true polymorphed gold dragons retain their ability to cast spells?

Question

I recently rewatched Mr. Rhexx's video on gold dragons and he mentioned that they love to practice magic. He also mentioned that all dragons can innately cast spells but gold dragons choose to further their magic prowess. Now true polymorph states that whilst morphed you can't cast spells unless your new form is capable of such. Now spell casting actually isn't present in the 5e gold dragon stat block so my question is if a mage was true polymorphed into a gold dragon would they still be able to cast spells?

Answer 1

The spell description of true polymorph explains this:

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.

This means that when polymorphed into an Adult Gold Dragon, you can only do what the Gold Dragon can do, and the Gold Dragon does not have spellcasting.

This is actually the case with all but one exception for adult dragons. The Adult Sapphire Dragon has innate spellcasting and can cast only scrying, telekinesis, and teleportation circle once per day each. To be clear, the Adult Sapphire Dragon can only cast the three spells listed on its stat block, and no others. Mr. Rhexx is referring to the optional variant rule for Dragons as spellcasters¹.

This search on DND Beyond gives a list of the Adult Dragons so you can review their stat blocks. They can also be found on pages 86-119 of the Monster Manual.

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¹Dragons are innately magical creatures that can master a few spells as they age, using this variant. A young or older dragon can innately cast a number of spells equal to its Charisma modifier. Each spell can be cast once per day, requiring no material components, and the spell’s level can be no higher than one-third the dragon’s challenge rating (rounded down). The dragon’s bonus to hit with spell attacks is equal to its proficiency bonus + its Charisma bonus. The dragon’s spell save DC equals 8 + its proficiency bonus + its Charisma modifier.

Answer 2

By default dragons are not innate spellcasters, but they can be

The stat blocks of the dragons in the Monster Manual don't have innate spellcasting, but there is an explicit variant called out in the MM to give any age of dragon spellcasting:

VARIANT: DRAGONS AS INNATE SPELLCASTERS

Dragons are innately magical creatures that can master a few spells as they age, using this variant.

A young or older dragon can innately cast a number of spells equal to its Charisma modifier. Each spell can be cast once per day, requiring no material components, and the spell’s level can be no higher than one-third the dragon’s challenge rating (rounded down). The dragon’s bonus to hit with spell attacks is equal to its proficiency bonus + its Charisma bonus. The dragon’s spell save DC equals 8 + its proficiency bonus + its Charisma modifier.

If your DM allows it you can True Polymorph into one of these variant dragons that can cast spells.

That being said, innate spellcaster monsters don't use spell slots to cast their spells. As a result you still won't be able to use your classes Spellcasting feature while True Polymorphed. This is due to this restriction in the True Polymorph spell:

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.