Loss and gain of region in ATAC seq
What you are looking for is "Differential accessibility analysis", you are using ATAC-seq, hence "accessibility". You will need to "call" and then "normalize" "count"(*) peaks, after these, you can use DESeq2 or edgeR for the "differential" part.

Here is a nice Nextflow workflow that addresses all of the steps of a differential accessibility analysis of ATAC-Seq data.

* featureCounts can be used the identify peaks, see this and this.