The code should be added in a separate file and then included in your functions.php
using require
.
Replace 1 from the code with the ID of the form that you want to dinamically populate with field values from ACF. You can find more info about the filters in the Gravity Forms documentation.
add_filter( 'gform_pre_render_1', 'populate_dropdown' );
add_filter( 'gform_pre_validation_1', 'populate_dropdown' );
add_filter( 'gform_admin_pre_render_1', 'populate_dropdown' );
add_filter( 'gform_pre_submission_filter_1', 'populate_dropdown' );