The easiest way is to have a variable set when your condition is met (ie. gameover=1), then use system set timescale: value.
[System:Compare variable (gameover=1):]
System Set TImescale 50
System Set Timescale 40
Or better yet, make a function that reduces a Timescale variable by 1 each .1 seconds and then simply call the function when gameover=1 and set timescale to the timescale variable so it grinds to a smooth stop. This would allow you to use a compare on the timescale variable as well so when timescaleVar=0 trigger GAME OVER.