The G4 was fairly matte. Probably the G5's, too.

I found this, maybe it'll help? https://www.amazon.com/2-Pack-Anti-Glare-Matte-Screen-Protector-Compatible-for-21-5-Inch-iMac-All-in-Ones-Desktop/dp/B01MXTZERJ

Of course, the benefit of Mac Pros is that you bring your own screen, which can be as matte or reflective as you want.