Not sure if anyone else has encountered this, but when attempting to use GPUs (RT) on united.com, I found an odd bug:

• searched for flights using advanced search and used the GPU upgrade option (economy to business)
• found flights with PZ available for my itinerary both ways
• selected the option for the lowest economy fare economy for a GPU (middle column) for the outbound and return legs
• on the checkout page, it showed an L fare (not eligible for a GPU) noting that an upgrade was available for the outbound (not expected). For the return, it showed a W fare with an upgrade confirmed message (expected). It was also pricing L + W instead W + W. So it definitely wasn't working as I wanted it to.

I went through the process two more times to make sure I selected the W/GPU-eligible option on the outbound, but it kept reverting the outbound to the L fare on the checkout page. I eventually went back to the search and put in "W" for the fare class to exclude anything below that, and that did the trick, thankfully, as I really didn't want to call an agent to make this simple booking!