Originally Posted by
writetorich
IF U is not confirmable should I apply a gold guest upgrade at time of booking?
You can't.
As for the rest of your question, if you buy a higher fare class, you will be higher in the upgrade list. It will help your chances, but no guarantees. It's your call as to whether it's worth the money.
What you can do is book the cheapest available, and then watch for U space. If it shows up, you can call in and change to a higher fare class and apply the GGU.
Of course the price might have gone up by then. This is where a tool like expertflyer is useful; you can see what the advance purchase restrictions of a particular fare are, and how much it will go up when it expires. You can also see availability on each booking class so you can see how much inventory is left in the class you want, and how much the next highest class will be if it runs out.