I know about gform_disable_css but I think I remember in the past there was a filter that would only disable the form input styles but maintain the layout columns. It might have been gform_disable_form_theme_css. Is there a new filter that will accomplish this?
Have you tried using custom CSS to override the default CSS code of Gravity Forms for the input fields or any other fields? Even if you utilize a hook, you will still need to apply custom CSS to implement a new style if you prefer not to use the theme’s style.