The "gap issue" is almost certainly not the reason for any shipping delay. Google support has indicated to customers that the small amount of gap exhibited on some devices is well within tolerances and won't impact the IP68 rating. And that seems to have been confirmed by at least one teardown which suggests that the gap may be cause by slightly too much sealant applied before the screen is pressed into the body.
It turns out that my own Pixel 5 is affected... and I never would have noticed if not for all of the noise about the gap issue. I can only see it if the light is just right and I hold the phone just so - and, of course, only if I remove the phone from its case. It's a complete non-issue for me.
I think
@ocnbrze's theory is more likely. Google always seems to have trouble meeting the initial demand following a device launch - and they seem to have been particularly impacted by COVID since they're still doing company-wide WFH, which slows everything down.