I own a GC2B binder and unless I find a shirt that is completely tight around my neck which is hard to find as I'm a small guy, more often than not my binder will show on my shoulder. I sometimes tuck my shirt around my neck under the binder but that looks a little sus and I don't like how it looks overall. I can't really think of a way to hide them, I've thought of like safety-pinning my shirt to my binder so it doesn't move past it but that doesn't sound very practical. Does anyone have any advice?

I usually pull it down as far as possible and tuck it into my underwear - if you can shift the straps at the top a bit further out it might help as well, although the effectiveness of that usually depends on your build.

I donate gc2b binders frequently to the transfolk around here.  The above tips are basically what my clients have done as well, actually. 

Was your chest size an even or odd number?

    xoxoxo Winter (15)

It might help to wear a button up/collared shirt. Or you could wear some kind of compression shirt (like under armor) underneath your regular shirt.

Probably not helpful but to the general public they'd assume you're simply wearing a normal vest. Having a strap showing wouldn't make anyone think your trans or that you look unusual. I have a GC2B and forget I'm wearing it when out.

Layering maybe? Find a tanktop that lines up with the binder, and wear it between the binder and your top layer.

Have you tried investing in strapless binders? They tend to help prevent any straps from showing.