There are a few things you need to understand:

First - *G benefit does not include "better" seat. Whether a *A airline decides to offer to *G is totally within their discretion. In short - you can ask, don't expect.

Second - As others have pointed out, Premium Plus is Premium Economy, which is a different service class from Economy. E+, on the other hand, is Economy with extra legroom. Your UA Gold entitles you to select E+ at booking, but nothing else. You are, as a matter of UA policies and practice, not entitled to any international upgrades unless the routes are short-haul (although the chance is still very slim). You will have to pay out for your international upgrades at all time.

Third - practically, each airline has established upgrade policies (published or not). The rule of thumb is the higher the status, the higher the chance. In short - your status is simply too low for any "free" upgrades you may hope for.

Last but not the least - your 170K M+ miles are not enough for upgrade, Mile upgrade usually requires a copay unless your fare class is high enough. Also - because a partner airline has been involved, more money and more miles may be needed.

In conclusion - unless you buy Premium Economy now, your chance stuck in Economy for the entire trip is close to 100%.