The GGU can only be applied to a K class fare or higher. The price you see is the lowest main fare class that has space in the bucket. Usually that is K class but could be higher. If the "F" is white with a solid blue backgroung, there is U space available and you can apply the GGU for an immediate upgrade. The process is to purchase the eligible Main fare and then you will be given the opportunity to apply the GGU after purchase and be upgraded into F after application.

If there are two or more segments in the itinerary and the connection time between them is 4 hours or less, a single GGU will upgrade all the segments that have U availability.

Personally, I would never purchase an eligible fare if I cannot immediately apply the GGU after purchase. The only advantage is that you would be higher on the waitlist for a complimentary upgrade without using a GGU if eligible. A GGU cannot be waitlisted so you would have to monitor for U space to open up which may never happen. Monitoring for U space is a manual process unless you use a subscription service like ExpertFlyer where you can set an alert to be notified via email if U space opens. If U does open you would have to call AS to apply the GGU.

The better approach is to purchase the cheapest fare, continue to monitor for U space, either manually or through the subscription service. Should U space open, you can call AS and "upfare" your itinerary to the eligible fare and apply the GGU. An upfare does not trigger a change fee.

The only disadvantage is that the lowest fare class eligible for GGU application may be higher than when you purchased the original fare. You can decide if the differential is still of value. It is better than purchasing an eligible fare and not ever being able to apply the GGU and still missing out on a complimentary upgrade, if eligible.

In reference to "if eligible," only Elites are eligible for complimentary upgrades. If the GGU was gifted and you don't have Elite status then you won't be eligible.

James