By adding the Survey module that comes with Gravity Forms, you can create star ratings. By default the star is pretty small but I found this code from GravityWP that makes them much bigger as pictured below.

Enter this custom CSS code:

.gsurvey-rating:not(:checked) > label, .gsurvey-rating:not(:checked) > label:hover, .gsurvey-rating:not(:checked) > label:hover ~ label, .gsurvey-rating:not(:checked) > label, .gsurvey-rating:not(:checked) > label:hover, .gsurvey-rating:not(:checked) > label:hover ~ label, .gsurvey-rating > input:checked ~ label {
    width: 65px;
    line-height: 50px;
    background-size: 50px 50px;
    color: rgba(221, 221, 221, 0) !important;
}