This snippet will change a selection in a radio button field when entering a value to another field.

change-radio-choice-selection.js

/**
 * Gravity Ranger // Gravity Forms // Change radio button choice when adding value to another field
 * https://gravityranger.com/gravity-forms-fancy-donation-form
 *
 * This snippet will change a selection in a radio button field when entering a value to another field.
 * Intended for a donation form with a price select field and and a user defined price field.
 *
 */

<script type="text/javascript">
  document.addEventListener('DOMContentLoaded', function() {
    // Get the price field
    var priceField = document.querySelector('#input_612_4'); // Price field: form ID 612, field ID 4

    // Get the specific radio buttons (choices) by their individual IDs
    var otherOption = document.querySelector('input#choice_612_14_4'); // Other choice (value other)

    // Add event listener for when the price field is manually changed
    priceField.addEventListener('input', function() {
      // Automatically select the "Other" radio button when the user types into the price field
      if (otherOption) {
        otherOption.checked = true; // Select the "Other" option
      }
    });
  });
</script>