get_posts showing only 5 posts. (nopaging and posts_per_page -1 not working)

The 'posts_per_page' => -1, and 'nopaging' => true, shouldn’t be in your tax_query params. That might be what’s throwing it off.

Try:

$args = array(
    'post_type'=>'gpp_prod',
    'orderby' => 'title',
    'order' => 'ASC',
    'posts_per_page' => -1,
    'nopaging' => true,
);

and:

$args['tax_query'] = array(
    array(            
        'taxonomy' => 'cat_prod',
        'field' => 'term_id',
        'terms' => $category
    )
);