Lets see, first make a common event called
Make it require a switch called "Timer On" and set it to parallel so it doesn't interrupt gameplay.
Create a conditional branch...if "Timer is less than or equal to 0 secs"
In true, add gameover command.
If false, leave that blank.
Also add a 5 wait frame so it doesn't lag.
Create the event now on your map that turns on timer, then after that turn on switch "Timer On"
Sorry for the sloppy instructions.
It should work fine though, let me know if theres a problem with it.