Hi,
One way you can restrict (and control) the budget going over in GP is to enable the POE (Purchase Order Enhancement) module in GP.. When doing so, you enable GP to control all the allocations coming from the purchasing to be checked against a given GL account thru a defined Budget ID.
This way when users enter requisitions and PO's in the system, it will require to be approved by someone if it's over-budget before it can actually be posted to GP.
Check the user help under Purchasing >> Setup >> Purchase Order Enhancements
POE is an optional feature in GP and you must be licensed to use it. Check with your partner if it's not already part of your current license agreement.