I've got no experience with cats, jassy, but I can tell you that it isn't uncommon for dogs to develop intestinal upset after neutering. Stress can upset the balance of organisms in the intestine--clostridium seems to be a common culprit--and we often have to treat with metronidazole after surgeries to get the dogs back on track. It can take a week or so for the balance to get out of kilter enough to result in diarrhea. So with luck, that's all that was going on with Gizmo!
Hope he's on the mend for good now!