When people want to pretend that communism is worse than capitalism because of gulags, that's not whataboutism.

Because when capitalism is imprisoning as many people as communism, and also leaving millions to live on the streets, starve, etc. then it's very relevant since you're comparing them.

When you criticize only one system for things done by both, pointing that out isn't deflection - it's simply showing the hypocrisy. When that's your only argument in the comparison, that's very relevant.