I was trying to use an upgrade online for my YYZ-SFO-HNL-GUM-HKG (one-way).
The website offered me choice of RPU and GPU/SWU to upgrade the 3 legs that are CPU/UDU eligible (i.e. not HNL-GUM, esp. since not a W+ fare), albeit waitlisting on 2 of the legs.

Q1. When I try to use the RPU I get an error (sorry, I forget exact text). When I used the GPU, it worked.
I called the 1K voice to see if the RPU can be applied instead, and was told it wasn't actually eligible on the GUM-HKG route and gave me a choice to leave as is, or cancel the GPU and use the RPU just for YYZ-SFO-HNL legs.
Is that correct?

Q2. On waitlist, does GPU have priority over RPU (and mileage upgrades) or is it purely by status and fare class as http://pss.united.com/web/en-US/cont...e/default.aspx indicates
i.e. is there an advantage to using a GPU instead of RPU to increase chance of upgrade (I won't be able to use all of mine this year, and I'm not paying up for the lottery)
My reading of that page says NO, that GPU and RPU are identical on the waitlist.

You should be able to use the RPU on the GUM-HKG route.

Your understanding is correct. Whether it is miles+copay, GPU, or RPU, all that matters is status, fare class, time added to the waitlist. Instrument type doesn't matter.