Will this trim install without removing my tailgate?

Install method not confirmed — test-fit the trim in your molding channel before committing to fasteners or adhesive. You may be able to slide it into place with the gate closed, but have your service manual handy to reference the factory channel setup.

Does this come ready to paint, or is it already finished?

Finish type not specified. If you want a custom color or protective coating, light scuff and prep with soap and water first, then apply your choice of exterior paint or clear coat.

Can this handle constant stepping without wearing through?

The textured surface is built to grip, but wear depends on how often you use it and what footwear you're wearing. Inspect yearly for cracks or separation, especially in cold climates where flex increases.