A permanent magical effect can be dispelled unless the spell explicitly clarifies that it cannot be. True Polymorph fits into this category, which means that it can be dispelled.

How do you counter true polymorph?

IIRC True Polymorph is a level 9 spell, so casting Counterspell at 9th level can stop it if they're there to witness it. If not, Dispel Magic might work.

Does Dispel Magic end true polymorph?

The Object (from Creature) will still retain the effect of True Polymorph upon itself, and if Dispel Magic is used on the object, the spell would end. If the object's hit points hit 0, the effect ends and the creature is returned to its original form.