You can add the event you want the variable to trigger to the same condition that change its value.
For instance, bonus add 50 to GB and do some other thing that you are expecting to happen every time GB value changes.
Or, if this is supposed to happen only once, you may try setting a initial value to GB, then set a condition that if GB is different from its initial value it will do whatever it is you want it to do.
Or moreover, you can work with two variables. GB and another variable to keep it's current value. Then if both GB and the second one doesn't have the same value you do whatever the function of GB is and by the end of it you set the other variable's value to the same value of GB.
There's actually a lot of ways to solve this, you should use that fits the best whatever it is you are trying to do.
I really hope this helps, if you need some further more assistance, please upload your .capx so we can have a better understanding of what you are trying to do.