Hello, Someone will correct me if I am wrong, but from what I understood, variants with AF=NA were simply not found in Gnomad.
On the other hand variants with frequency were likely found among gnomad samples, but deemed as poor quality and therefore removed.
Regarding the genome/exome, it depends on the version of gnomad browser you are using. Gnomad v3 only provides information from hg38 aligned whole genome data, while v2.1 provides information from hg19 aligned whole exomes and a few genomes.
Cross-posted on bioinfo SE: https://bioinformatics.stackexchange.com/questions/20181/allele-frequency-gnomad-data where it's received an accepted answer.
Bad form, OP. Very bad form.