Planet Gravity can be implemented nicely by modifying CharachterMovementComponent,and there are dozens of methods for you to override.
Planet mesh?Maybe generating some procedural mesh is ok.
But we can always start with static mesh to give us some confidence.
Then, try some voxel plugin or just planet-lod algorithm to build your planet.
My test:
https://youtu.be/cBlZWSHDaE4