Let me be a bit more specific: I want this to change programmaticaly. Gantry 5 uses the H2 within the <div class="page-header">. This should be changed to H1 and will require a template specific adjustment.
In the Joomla world H1 is reserved for the display of the page title.
To change this you need to add an override for com_content.
Gantry 5 is not different than regular templates like beez and you can follow the Joomla docs here: