Part 1: First you have to find the Item ID of the premium piece of content you want to let the Sales Team know your Lead/Person has viewed
1a. Edit the Item in the Manage Stream page
1b. Find the longest number in the Browser URL (<HubId>/<ItemId>/StreamId>). This is your Item Id you'll need to target in your Marketo Campaign Smartlist
Part 2: Setting up the Marketo Smart Campaign
2a. Campaign Smartlist to look for Data Value Changes on uf_conversion_item_id. The uf_conversion_item_id will update with the Item Id of an asset when a Lead/Person submitted a Form CTA on or when a fully profiled Lead/Person views an Asset where the usual Form CTA hides.
2b. Create the Interesting Moment that lets your Sales Team know the Lead/Person has viewed this premium piece of content. This will show up in the Leads/Person's Marketo Activity log and within the Marketo Sales Insight module within Salesforce!
Do this for each of your gated assets!
Create a Marketo Smart Campaign for each of your premium content pieces with a Data Value change looking for those Item Ids, and your Sales Team can easily keep up to speed on what content their prospects are consuming!