Demonstrates how to pre-populate checkbox fields when using the Gravity Forms plugin for WordPress

functions.php

<?php

# Make sure to replace {id} with your form's id
add_filter( 'gform_pre_render_{id}', 'my_populate_checkbox' );

function my_populate_checkbox( $form ) {
  
  /**
   * Loop through form fields
   *
   * Note we are using the `$field` array as a direct reference using `&`. 
   * This means that changing its value will within the loop will 
   * change the corresponding `$form` array item
   */
  foreach( $form['fields'] as &$field ) {
    
    # Make sure to change `1` to the ID of the checkbox field that you want to pre-populate
    if( 1 === $field->id ) {
      
      /**
       * Loop through the choices for this checkbox
       *
       * Note again that we are passing `$choice` by reference in order to change the 
       * corresponding array item within the `$field` array
       */
      foreach( $field->choices as &$choice ) {
        
        /**
         * If this choice has a value of 'red' or 'blue', then make sure the checkbox is pre-checked
         * by setting the `isSelected` parameter to `true`
         */
        
        if( 'red' === $choice['value'] || 'blue' === $choice['value'] ) {
          $choice['isSelected'] = true;
        }
      } # end foreach: $field->choices
    } # end if: $field->id equals 1
  } # end foreach: $form['fields']
  
  # return the altered `$form` array to Gravity Forms
  return $form;

} # end: my_populate_checkbox