For international travel the published priority list is -- status of instrument-holder; then fare class; then time applied.

If you apply a 1K member's GPU to an international Y/B/M fare, this should have higher priority than a 1K member's GPU on any other fare class (H or lower), and it'll also have higher priority than a non-elite member's mileage upgrade applied to any fare class.

That being said, if you hold a Y or B fare and plan a bunch of international travel, consider a mileage upgrade; the cost in miles can be fairly low and you can save a GPU for a later W fare.

For domestic travel things are a little special; Y/B fares are super high even without an instrument.