In what ways can I achieve gender equality?

You can select it in the Game Rules when setting up a new game.

There also may be a chance that another player (AI or human) reforms their faith to have gender equality, and then you could convert to it.

While there are no Female-Dominated religions in the base game, certain African and a subset of Eastern faiths support equality. Beyond that, a few of the Christian heresies that pop-up have gender equality as a core precept.

What's going to be available is highly dependent upon where you've got diplomatic reach to, as the fastest way to switch would be to marry into a religion (by adopting the faith of your spouse).

Here's the full list (as of 1.3.X / Corvus):

Christian

• Bogomilism
• Catharism
• Messalianism
• Adamitism

Islam

• Azraqism
• Sufrism

Eastern

• Nangchos (Buddhist)
• Vajrayana (Buddhist)
• Srikula Shaktism (Hindu)
• Kalikula Shaktism (Hindu)
• Yapaniya (Jainism)
• Khurramism (Zoroastrianism)
• Mazdakism (Zoroastrianism)
• Taoism (All Variants)

Others

• Cainitism (Dualism)
• Tibetan (Both Variants)
• Vvluphixje (Hsexje)
• Donyi-Poloism (Tani)
• Yumaism (Mundhumism)
• Bori (Hausa)
• Orisa (Yoruba)
• Roog Sene (Senegambian)

In addition to this, you can change your realm's succession laws, but you can only make things stricter e.g. "Male-Preferred" or "Female-Preferred" in an Equal faith (but not "Male Only" or "Female Only").

Finally, as OrangeDog mentioned, you can set the default gender laws in the game creation screen. While setting the default to "Equal" won't work with Ironman games (if you're trying for achievements, for instance), you can "Invert" the defaults (so the vast majority become Female-Dominated instead) while still allowing Ironman.