I was thinking this too! Gait recognition can completely bypass facial coverings as a means of identification, but I also don't think it'll be much help here.

Gait recognition can be bypassed by things as simple as putting a rock in your shoe so you walk differently, so when you think about how much extra heavy gear, different shoes, and different overall movement patterns ICE agents will possibly be engaging in, it might not hold up well at tracking them down, especially since to recognize someone by gait, you'd need footage of them that you can already identify them in, to then train the model on.

In the case of fucklapd.com, this was easy because they could just get public record data for headshot photos, but there isn't a comparable database with names directly tied to it for gait. I will say though, a lot of these undercover agents might be easier to track by gait since they'll still generally be wearing more normal attire, and it might be more possible to associate them with who they are outside of work since it's easier to slip up when you're just wearing normal clothes.

A lot of masks only work in the visible light spectrum. It's entirely possible to "radar" images and remove them.