We have recently installed a new shower Basin and toilet. The new shower/toilet is infrequently used. In spite of daily airing with good ventilation there is a strong unpleasant smell. It appears to be coming from the basin drain and possibly the floor level shower drain.
Could this be a failure of the plumber to put in goose necks

Could this be a failure of the plumber to put in goose necks I can't imagine any plumber installing a fixture without a trap. You may check by inserting a ride down the drain. If he comes back with a few inches of water on it. You have a trap. Are you sure that the water hasn't evaporated from the traps do to in infrequent use? Let me know what you find. Back to you, Tom