Gabriel a travaillé comme comptable pendant dix ans

Bonjour Gabriel,

Here, the English sentence uses "worked" which is the equivalent of the English simple past, meaning the action is a single completed past action. You cannot translate that tense with Le Plus-que-Parfait (i.e. avait travaillé = had worked). 

Take a look at the links below:

Using the French compound past (Le Passé Composé) to express sentences in the English simple past

Le Passé Composé (Indicatif)

Simple Past

Le Plus-que-Parfait (Indicatif)

I hope this is helpful.

Bonne journée !