Show all wp_get_post_terms slugs

This is more of a PHP question, but the solution is simple – you need to use a foreach-loop on $getslug, because you just echo the slug of the first taxonomy.

The function wp_get_post_terms() does not return a single object, but an array of objects. You are right with the [0], this indicates that you are checking the first entry of said array.

Your function should look something like this:

$getslugid = wp_get_post_terms( $post->ID, 'opd_taggallery' ); 
foreach( $getslugid as $thisslug ) {

    echo $thisslug->slug . ' '; // Added a space between the slugs with . ' '

}