Gravity Forms used to not correct the invalid HTML, so you could have the opening div in one HTML field, and a closing div in another, and style everything within that div, but validation was added at some point and open divs will be closed, so that hack no longer works.
The closest I know of to this is Collapsible Sections from JetSloth:
It allows you to specify whether a Gravity Forms Section field should be the Open or Close of a section. I know the collapsible part is not what you’re looking for, but their approach could be replicated with some custom code.
I don’t know of any other methods to do this without custom code.