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
)
);