For the record the word 'niger' is the Latin nominative masculine singular for the colour black. It is pronounced with a long 'i' as in 'neeger'.
Gibson would have learned Latin at school so given that the dog was masculine the literal translation of 'Nigger' in the context he used it would have been 'black dog'.
There is no suggestion that the term was used was used as a derogatory reference to African Americans.
Had the dog had white fur he might have called it 'Albus' - 'white dog.
If it had had brown fir he could have called 'Fulvus' or 'Spadix'.
If it had been auburn or redidish brown he might have called it 'Zingiberi' but that might have raised a few eyebrows in the Mess is it's a neuter noun and could have attracted some banter.
Obviously, in today's terms that could be seen as offensive to red heads and that's really dodgy as they can be really feisty about having their race treated humorously as I know being married to one!