SpaceJim has a pretty good handle on space! (Surprise, surprise)

Just to embelish, you don't need to do a trace at all to determine gravity.
just use a vec_dist command on the player.pos and planet.pos to get the distance.

From the distance away, derive the force of gravity.

Take the force, and convert it to a vector.

Use the vec_rotate command to point your gravity vector at the planet.

apply this rotated force to your entity being influenced by gravity.

I'll leave the details to you, but I hope this gets you started!

- WildCat