Pardot is a software as a service marketing automation platform by Salesforce that offers email automation, targeted email campaigns, and lead management for B2B sales and marketing organizations.
As this article explains, Gearset will work for any cloud that builds on the core Salesforce platform using the Salesforce metadata format — including Sales Cloud (the core CRM of Salesforce), Service Cloud, and the Salesforce Platform.
However, Salesforce offers Pardot as part of Marketing Cloud, with the latter having been acquired by Salesforce. As such, Pardot (and Marketing Cloud) has historically not been built on the Salesforce platform, but instead on a different underlying infrastructure, with distinct data and metadata. For this reason, Gearset won't work with Pardot. (Other Salesforce clouds that Gearset doesn't support include IoT Cloud and Commerce Cloud.)
Having said that, as this article explains, over the past few years "Pardot has been moving across to the core Salesforce platform, piece by piece." The long-term aim is to "have all Pardot functionality housed on the core Salesforce platform." If that were to happen, then Gearset support would follow.