ATACMS are slightly faster, > Mach 3, than GMLRS, >Mach 2.5, per open source information, and a bit larger. They would probably be a bit easier to detect on radar, but I think it's pretty safe to say that if they aren't shooting down GMLRS, they probably won't be shooting down ATACMS.
Now if they concentrated their air defense for a particularly high value target they have a chance of shooting one down, if you go by the theoretical or declared capabilities of Russian AD systems. However as we have seen the real world Russian performance typically does not measure up to theoretical capabilities.
Now if they concentrated their air defense for a particularly high value target they have a chance of shooting one down, if you go by the theoretical or declared capabilities of Russian AD systems. However as we have seen the real world Russian performance typically does not measure up to theoretical capabilities.