It ’s similar to “おつかれさま(O Tsu Ka Re Sa Ma”, but it ’s rude to use it for a senior person.
“ごくろうさま(Go Ku Rou Sa Ma)” is a negligent word used by the junior person for the senior person, so it is rude to use it for the boss as well as outside the company.
The meaning is the same as “おつかれさま(O Tsu Ka Re Sa Ma”, but the nuance of “ごくろうさま(Go Ku Rou Sa Ma)” is negligent word to the person who did something for someone, so this is a general manner not to use it for superiors.