Is this the correct approach to accept dynamic fields in Gin?

It is a way of handling JSON objects with unknown names, but not necessarily the correct way. For example, if the know the the object's values all map to Go type T, then you should use var data map[string]T or var data map[string]*T.

Are there any limitations or best practices I should be aware of when binding to a map[string]interface{}?

The limitation is that you must access the map values using type assertions or reflection. This can be tedious.

How can I validate fields or types if I don’t know the keys in advance?

If you know that the object's values correspond to some type Go type T, then see part one of this answer.

If you don't know the object's names or the type of the object's values, then you have no information to validate.