This might be a stupid question, but what’s the “proper” way to hold the neck while playing an open G? Everything I’ve tried feels awkward.
If your thumb is pushing the neck away while playing open strings, it sounds like you are pushing too hard with your thumb 
I just hold it with my hand open, fingers hovering over the frets, not touching anything. Thumb on the back as usual.
What @howard said… or, you could choose to play the G on the 5th fret of the D string Might give you more control anyway…
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Well, depends. Lots of times that won’t make sense. Open strings are useful too. Best to fix the issue he’s having.
(in this case it would be fine though yeah - but he should still find out why this is hard for him)
Yeah, you’re right @howard. I guess I’m biased because I’m adverse to playing open strings and avoid it as much as possible.
Are you using a strap with your bass?
Yeah I definitely do use more thumb than I should… I’ll try to reduce the pressure, thanks.
Yes, of course! And it also depends where his fingers have to go after that phrase shown in the example.
Generally, I am with @PamPurrs here and try to avoid open strings when possible. But, they do come in handy and are awesome when you can let them ring 
Yes but I play sitting down with the bass on my leg, so the strap is slack and not doing much
You could try adjusting the strap so that it is under tension as well while sitting. That would prevent the neck from moving around too much without having to keep it in place.
I have my straps tight enough to take most of weight. If I remember correctly, I think Josh talks about it in the course.
This is as short as the strap goes unfortunately. I’ll probably buy a shorter one sometime to see how that feels.
Upon further research it’s not my left thumb pushing away the neck, it’s doing it on its own. Without my fretting hand holding it in place, it just swings into the position it wants to be in. Which is a couple of centimeters away from me.
Did you solve it?
Don’t remember what I ended up doing, but I no longer seem to be having this problem.
Honestly I’d just tell 3-years-ago-me to stop overanalyzing things and just keep practicing. Your hands and body need time to get used to handling a bass, and that’s all there is to it really.