I think out of all the sequels/prequels he had input only on Alien3, and Prometheus.
His time on Alien3 was relatively short - weeks? A month?, and he only got that on the insistence of director David Fincher. Prometheus was much less than that. In real terms; 'Hey, Giger pop down to the studio and see what we are doing, and can you design us a mural whilst you are at it..?'