As requested, the following example VMF shows how to set up NPC spawns (with GM buttons) as seen in current Overwatch levels. It'll also show how to implement some traps (manhacks, turret) and the included txt file explains how to scale unit spawns based on the number of active players on the server.
This should have all the information you need to get the level working for the GM.
Again, keep in mind that NPC spawns will be replaced by a new Barracks system (custom entities) in 1.4.0, which will make it a lot easier to set up NPC spawns. The current entities will remain unaffected (old GM spawns will still work fine, your map won't break if you use them).
Let me know if you have any questions.