To explain, this usually happens when Squarespace have attempted to send a form submission to the email address in the past, before it was fully set up and accepting mail, and it failed to send. In such circumstances, Squarespace log the failure to send the email (it 'bounced') and they mark the email address as 'bad'. This prevents them from sending further (costly) emails to email addresses that don't accept them.
Check in with Squarespace Customer Care to see if the gmail address has been added to their form 'bounce list' as this would prevent it from receiving the form emails. If it is on the list, they can remove it.
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