I found code to populate dropdown, but the question is :
what is the hook function to display a dropdown instead of simple textbox for a column ?
The code to populate :
add_filter('gform_pre_render_5', 'populate_dropdown');
function populate_dropdown() {
global $wpdb;
foreach( $form['fields'] as &$field ) {
// update "populate-dropdown" to dropdown(select) CSS class
if($field['type'] != 'select' || strpos($field['cssClass'], 'populate-dropdown') === false)
continue;
// our dropdown options
$choices = array(array('text' => 'Select a Post', 'value' => ' '));
// select our custom fields
$posts = $wpdb->get_results( "SELECT ID, btc_state_short FROM btc_state_list" );
foreach( $posts as $post ) {
$choices[] = array('text' => $post->ID, 'value' => $post->btc_state_short);
}
$field['choices'] = $choices;
// if this do nothing, try
// return $field (uncomment if neeed)
}
}