GTR fares are not eligible for immediate upgrade to premium class (row 6-10 on Boeing) for Gold.
Details are here:
https://www.alaskaair.com/content/mi...rship-benefits
To explain the F Upgrade fare being more expensive:
Instant upgrade fares are based on the
highest published economy fares (YSBMZ). You aren't purchasing First Class - you are purchasing an expensive economy-class ticket and getting a free upgrade to F. It is designed this way to reward customers who have to purchase the most expensive fares (travel policy, booking late, etc).
Alaska will sometimes run first class sales (sold as P fares) which are not necessarily based on the expensive economy fare price (those don't really go on sale). This creates a situation where the upgrade fare is more expensive than the F fare.
The U availability is a separate issue too:
Since the VX acquisition, they seem to be withholding U availability because of the Airbus with 8F seats being in the fleet now. Many times, I've seen an F cabin "sold out" only to have 8 people upgraded at the gate. They are probably trying to avoid an oversold F cabin in case an airbus gets swapped in.