It's hard to rate a work that is of valid historical value yet, as a first-person narrative, is highly colored by not only the viewpoints of its author but the fact he was only able to write from his own cultural vantage and was unaware of most facets of the Inka beyond what the Spanish had discerned in their short time exposed to Inkan culture.
That all said, there is no doubting that an early account like this provides details that would otherwise be unobtainable, even if the narrative is greatly slanted. Once you get beyond the overwrought period language of de Gamboa—which rivals anything you'd expect in the worst Disney version of colonial dandies who think too highly of themselves, but is the real thing—you'll encounter some illuminating, even surprising, insights at times. However, it greatly helps to approach this work with a good grasp on what we know of the history and anthro-sociology of the Inka nowadays.