Okay, looking at this a bit deeper. I think I have a hunch as to what’s going on.
curl’ing that page directly contains
<form novalidate name='Retmes_Contact' method='post' action='/success' class>
<input type='hidden' name='form-name' value='Retmes_Contact' />
Which appears to be a post-build form that Netlify transformed to remove the data-netlify
fields and add the hidden form-name
field for. Nice.
But in the DOM, post render / Gatsby ‘stuff’, I see
<form novalidate="" name="Retmes_Contact" method="post" action="/success" class="">
which is very close but I’m presuming that the static form from build-time actually gets removed and this form is a fully separate object just posting to the same place. The one thing you’re missing in this ‘dynamic’ form is the hidden field. Add that exact hidden field to this dynamic form and I bet it’ll work perfect:
<input type='hidden' name='form-name' value='Retmes_Contact' />
Oddly enough, in reviewing your form on the Retmon side, there’s no hidden input there either… so you should add one… but typically I see that forms without that input don’t make it to any form box, let alone cross-wired to a single one. Worth additional digging and sandboxing in an isolated repository or two if adding those two elements to your sites fixes the issue 
btw: docs for reference, pegged directly at the spot you need(ed) https://docs.netlify.com/forms/setup/#work-with-javascript-rendered-forms