When you put "choose(1,2,3,4,5,6,7)" inside quotation marks as you have, it is no longer an expression, but a string.
I don't think you can have an expression inside a variable. Are you trying to have the random number picked when you go into the next layout? Or have the next layout pick the random number?
1. Use an action to set your GLOBAL_MONSG variable to choose(1,2,3,4,5,6,7), in which case a random number whole number between and including 1 and 7 will be stored in the variable. Then you can set RandomMonster to GLOBAL_MONSG and get the same number.
2. Use a function that returns "choose(1,2,3,4,5,6,7)", and have your action set your RandomMonster variable to your function output.