It's a gamble.

My GPU upgrades have cleared 90%+ over the last few years. I typically pay an extra $1-300 USD each way for W+, but I get a lie flat seat 90%+ ($2-4000 in value) of the time. So it is a good deal compared to buying business, or using miles + copay.

If you are looking at the bottom line, and don't consider business to be worth hundreds of dollars more on a long flight, you have to do a lot more work.

1) Look for mid-week, off-business times with available R (use expert mode on the UA website). If you buy a W+ with R > 0, you get a 100% upgrade, so it is not a gamble.

2) Save the GPUs for trips on higher demand days, or with shorter notice, so the extra cost of W+ is not too high.

3) Use them domestically, latin america, Caribbean, Hawaii, etc. Much easier to find R > 0, you jump the CPU queue, and there is no extra cost to using them.

4) If your GPU doesn't clear by T-24, look for a SDC (same day change) with either more unsold business (J/C/D/Z/P) or available R (often found within T-24). Changing to these flights will get you an upgrade (may need to call to cut the line when R > 0, see other threads) or increase your chances.