ge_gameplay $scenarioname (e.g., ge_gameplay yolt) is the cvar that deterimes which scenario is active.
deathmatch.cfg is the script that controls the cvar settings when the deathmatch scenario activates and is availiable so you can specify things like paintball, round times, etc. deathmatch.lua is the script that handles the scenario rules and behaviors. It is encoded in public releases and is not to be altered. However, if you are missing the file or did not specify the name correctly, obviously it will not function.