Use custom marker when specific checkbox is checked
I would use CSS for this.


Add your featured checkbox field
Add a views-row class, in the format settings, with the replacement pattern of the value for the checkbox


Now I can't remember if GMap will add that class to the marker rows too. If it does not, I would advise you to override the template for the view and in the class section, add $fields['field_featured'], making sure that there is a space before it


Using that class, you can style it however you want using CSS and/or JS


Hope that helps!