Gravity Forms - Set a radio selected button via PHP

filter-example.php

add_filter('gform_pre_render', function($form)
{
    $user_consent_form = get_field('user_contact_preferences_form', 'option');
    if ($user_consent_form) {
        if ($form['id'] === (int)$user_consent_form) {
            // This should return an array if its set.
            $user_consent_marketing = get_user_meta(get_current_user_id(), 'marketing_consent');

            if ($user_consent_marketing) {
                // find out which field is "marketing_consent"
                foreach ($form['fields'] as &$field) {
                    if ($field['adminLabel'] === 'marketing_consent'
                        && $field['type'] === 'radio') {

                        if (is_array($field['choices'])) {
                            $replaced_choices = [];

                            foreach ($field['choices'] as &$choice) {
                                if ($choice['value'] === $user_consent_marketing[0]) {
                                    $choice['isSelected'] = true;
                                }
                                $replaced_choices[] = $choice;
                            }

                            $field['choices'] = $replaced_choices;
                        }
                    }
                }
            }
        }
    }
    return $form;
});

Should be noted that in the second foreach for choices if you don't use a $replacement_choices array and resave this against the $field['choices'] it wont end up being set in the final return.... took me too long to realise this.