It has become fashionable in recent years to base the appeal of a novel on personal identification with the protagonist. That is, enjoyment of the novel is supposed to consist in a personal aspiration to be the protagonist or in a close personal identification with the protagonist (recognizing oneself in the protagonist). Such novels inherently have a strong gender bias. Men rarely aspire to be or see themselves in female characters, or vice versa. (Not never, perhaps, but rarely.)
But this is not the classical appeal of a novel. The classical appeal has always been one of observation. The pleasure comes not in identifying with but in getting to know, or at least getting to observe and understand, the protagonist. This kind of appeal is, if you like, objective rather than subjective. It is a pleasure in looking outward rather than a pleasure in looking inward.
Such a novel has far less of a gender bias than one that depends on identification with the main character. There are a huge range of things that both men and women like to observe, a huge range of people that both men and women like to meet. A novel of observation can appeal to both sexes equally in a way that a novel of identification cannot.
This is not to say that there are no differences in the subjects that men and women are objectively interested in, because of course there are. There are subjects that far more men are interested in than women, and vice versa, so if your novel of observation deals with one of these subjects, it will skew your audience accordingly.
Finally, a novel of observation may be judgemental or may withhold judgement. A judgemental novel appeals more strongly to those the support its judgements, but repels those who do not support them. A novel that withholds judgement may appeal to both sides of an issue. Man and women tend to judge differently on a number of issues, so if you write a judgemental novel, the judgements you express will tend to bias your readership to one gender or another.
So, if you want to appeal equally to both sexes, you would do best to write a novel that is observational, non-judgemental, and deals with a subject of equal interest to both sexes.
Not that you should expect to achieve perfect gender balance. If tales are to be believed, more women than men read novels, and their tastes are more catholic, and so you would probably have to skew the appeal to the male side to achieve an actual gender balance in your readership.
Rather than pursuing gender balance, therefore, I would suggest attempting to write an observational non-judgemental novel simply on the grounds that this is the purest form of the art, the one most free of pandering, the one most worthy of the artist.