Hello WPMonks,
I believe there might be a conflict in the recent update for the Styles and Layouts Gravity Form plugin. As the others have mentioned in this post, when you try to save the Font size using customizer you will get the error message (see link below).
View post on imgur.com
I did try to deactivate the Styles and Forms plugin in order to save, which it does save, but that is not an optimal solution as all of the edit options including the link to Styles and Layouts within each form is gone.
This is a fantastic plugin, great job this. Hopefully you can reproduce this on your end and then identify the conflict and release an update. Cheers mate!