I was pretty terse! Let me unpack that a bit.
I think that GTD intentionally embraces the notion that "important" and "not important" basically have no useful meaning.
Here's some of the reasons I say that:
This method is significantly different from traditional time-management training. Most of those models leave you with the impression that if something you tell yourself to do isn't that important, then it's not that important -- to track, manage, or deal with. But in my experience that's inaccurate, at least in terms of how a less-than-conscious part of us operates.
If the thing's not important enough to be done, *throw it away*. If it is, and if you're going to do it sometime, the efficiency factor should come into play.
If you don't decide what needs to be done about your secretary's birthday, because it's "not that important" right now, that open loop will take up energy and prevent you from having a totally effective, clear focus on what is important.
So, yes, I am glossing over some of the subtleties of GTD. But I think it's fair to say on a gross level that there's two categories of importance:
1. Things that you feel are important enough to do. They are in the system.
2. Things that you feel are not important enough to do. They are in the trash.
And that, on some fundamental level, really everything in the first category has a sort of equality to it. They're all things you want to do As Soon As Possible.
So, to return to some of the earlier examples, I'm inclined to think that you absolutely should do 'go to the gym' or 'learn Spanish' whenever it's easiest for you to do those things. If it's easiest to go to the gym after work on Mondays, then follow that path. I don't think this is rocket science.
Upon further reflection, I'm starting to think that perhaps some people are using these "priority codes" as essentially a motivational tool. I personally don't think they're especially well-suited to that purpose, but I can allow that maybe for some people they work well in that capacity. But it gets a bit confusing to suggest that they have anything to do with priority or importance at that point.
So... yeah. Waking up in the morning and thinking "today I want to clean out the garage" isn't problematic and is probably useful. But I think it's a distinctive thing from 'importance' or 'priority', although goal-setting is of course informed by those things.
Hmmm. This rabbit hole goes deeper than I suspected. Thanks for this topic! Good food for thought.