Gravity assists primarily work when you are inbound at speed to the body you'll slingshot around, not struggling to reach it's altitude from beneath (i.e. you a

Gravity assists primarily work when you are inbound at speed to the body you'll slingshot around, not struggling to reach it's altitude from beneath (i.e. you are lower in Jupiter's gravity well than the moon, because you are launching into space from a stationary start).