That socks... Sometimes we try our hardest and still we feel we must have done something wrong when they die.. But I bet you tried your best to help him, and I bet he is glad for everything you have done for him.
So don't feel bad, you still have Gilligan, love him as much as you can! And when that loss-pain finally eases up a bit, try again with a new male, that's the joy of life, even though one leaves us without saying goodbye, a new one appears to warm our hearts once again.
May him rest in peace.