people.js

const people = {
  bob: {
    age: 15,
    gender: 'male',
  },
  jessica: {
    age: 24,
    gender: 'female',
  },
  lucy: {
    age: 11,
    gender: 'female',
  },
  sally: {
    age: 14,
    gender: 'female',
  },
}

const { males, females } = Object.keys(people).reduce(
  (acc, name) => {
    const person = people[name]
    if (person.gender === 'male') {
      acc.males.push(name)
    } else {
      acc.females.push(name)
    }
    return acc
  },
  { males: [], females: [] },
)

console.log(males) // ["bob"]
console.log(females) // ["jessica", "lucy", "sally"]