GC normalization ATAC seq data
The only reason to normalize for GC content in RNA-seq is if it differs notably between samples/groups. If that's not the case and you aren't trying to compare genes withing samples then you have no reason to try to account for GC content. The same goes for ATAC-seq, though there the danger with trying to normalize for GC content is that you then mask changes in global accessibility.
In short, normalize for things that should cause problems with your analysis and nothing else.