Players can choose which characters they want to control and bring on missions in Gotham Knights. As of now, we haven't heard whether players can replay single-player missions, but multiplayer is a different story.
Can I replay Gotham Knights missions in co-op?
You can invite friends in and out of your session at any time, and you can assist other players with whatever they need, including missions. Gotham Knights executive producer Fleur Marty notes in game's official Discord page that if you have already finished a mission that your friend will tackle, you can still replay it together in co-op. Furthermore, if you help a friend complete a mission that you haven't yet completed, then you have the option of skipping the mission in your game when you encounter it.
Can I keep all my stuff if I join another player's game in Gotham Knights?
Creative director Patrick Redding and Marty met with IGN in August 2020 and spoke on character customization. Players can keep everything they earned in their game session. You keep your gear, your level, and anything else that made your Knight unique.
For more on Gotham Knights, check out Gotham Knights shows off gameplay of Nightwing and Red Hood or How does Gear work in Gotham Knights? here on Pro Game Guides!