Your gaslighter may have personality traits, or a personality disorder, which is not narcissism but which stems from a point of past trauma and fear. For instance, if your partner has abandonment issues, they may find all kinds of manipulative ways of making you stay with them—including gaslighting. In this case, it could be that their behaviour only comes to the fore when they feel particularly threatened and have found themselves in a situation which has triggered a strong sense of fear.
If your gaslighter is someone who hates confrontation, and is far more likely to find passive-aggressive ways of dealing with issues, they may resort to gaslighting instead of being open and honest with you. Part of someone’s need for conflict avoidance and the resulting passive-aggressive behaviour may stem from the fact that it was unsafe when they were growing up to openly express an opinion or to be honest about their needs without this descending into conflict of an unsafe nature.