GLCID for Google Adwords conversions through Marketo Forms

So as we understand it - passing that GLCID (the Google Click ID) is key to measuring offline conversions and associating that back to particular ad clicks in Adwords. Natively, Google Analytics knows online conversions, but since we do considerable volume offline (and that volume is no doubt biased towards certain products / ads) we want to supplement the online conversion data with the offline, giving a true picture of return on ad spend on an ad-by-ad basis within Adwords.

So if we can pass through glcid through a Marketo form to Salesforce, then we can link it with offline conversions / phone calls, and export back into Adwords.

The main problem is that if they hit a landing page from the ad where there is a Marketo form, we could capture it as a hidden form field. Easy. But since users navigate all around the site and might only get to a form fill after a few pages, that query parameter doesn't appear in the URL anymore - so I don't know how to populate it in the form. I'm guessing it lives in the data layer, so maybe with some custom Javascript?

Here's an example of a URL with the glcid, after an Adwords ad click - note the last parameter, that's what I'm after. That ties a user 1:1 with an ad click. 

Any ideas? Thanks!