He did indeed die recently. I hadn't seen the kudu at all in a while (and they're currently listed as off-exhibit on the website), so I contacted the zoo about them and today they replied with this:
"One individual is recovering from an injury, one is being permanently separated from the group, one individual died.
We do hope to have the mom and calves back on exhibit starting July 26 but that will be determined that day during a follow-up vet check."
Now while they didn't say Garrett specifically died, they had six kudus, and they mention the mom and calves which is four of them, and today I saw Machi on exhibit alone with the hornbills.