Use zcat and sed or awk to edit compressed .gz text file

I wrote a script called zawk which can do this natively. It's similar to glenn jackman's answer to a duplicate of this question, but it handles awk options and several different compression mechanisms and input methods while retaining FILENAME and FNR.

You'd use it like:

zawk 'awk logic goes here' log*.gz

This does not address sed's "in-place" flag (-i).

You can't bypass compression, but you can chain the decompress/edit/recompress together in an automated fashion:

for f in /dir/*; do
  cp "$f" "$f~" &&   
  gzip -cd "$f~" | sed '2~4s/^.\{6\}//' | gzip > "$f"
done

If you're quite confident in the operation, you can remove the backup files by adding rm "$f~" to the end of the loop body.