Gridiron is the new objective-based gamemode in Operation 3. 5v5 Capture the Flag with one life.
To capture the flag, either pick it up where it spawns, grab it from a dead teammate, or kill an enemy carrying it. Run it to the Endzone, and hold it for 5 seconds to score and end the round. Be careful and communicate with your team, because if the other team is eliminated the round will end regardless.
Currently, this is doable in a private match. If this gets patched or changed, I'll edit the solution. Thanks x The Horrorist!
Let me know if I missed anything that can improve the guide!