How to call a variable again GML

I have a variable called global.goldchance = irandom(99);. I use it to get a random number from 0 to 99. However, I need to be able to run it again so that it chooses a different number. Writing var goldchance = irandom(99); doesn't work because the variable will be destroyed afterwards. Writing the variable again won't work because that's not calling it again; that's simply stating it. How do I do this?