"De reiste gjennom hele landet."

Translation: They traveled through the whole country.

[PaoloLim]

Is "De reiste gjennom det hele landet." acceptable as well?

[Ingebj]

No, you need to omit "det", really.

[fveldig]

For some adjectives, like 'hele' and 'halve', you should avoid using 'det' or 'den'.